Well, you don't specify if it's a 32 or 64 bit dll and you don't include the error message, but I'll guess that it's the same issue as described in this KB article: Error Message When You Run Regsvr32.exe on 64-Bit Windows

Quote from that article:

This behavior occurs because the Regsvr32.exe file in the System32 folder is a 64-bit version. When you run Regsvr32 to register a DLL, you are using the 64-bit version by default.

Solution from that article:

To resolve this issue, run Regsvr32.exe from the %SystemRoot%\Syswow64 folder. For example, type the following commands to register the DLL: cd \windows\syswow64 regsvr32 c:\filename.dll

Type regsvr32 name.dll into the Command Prompt (executed in elevated mode!) and press "Enter." Note that name.dll should be replaced with the name of the DLL that you want to register. For example, if you want to register the iexplore.dll, type regsvr32 iexplore.dll.

Knowing the error message would be rather valuable. It is meant to provide info, even though it doesn't make any sense to you it does to us. Being forced to guess, I'd say that the DLL is a 32-bit DirectX filter. In which case this should be the proper course of action:

cd c:\windows\syswow64
move ..\system32\dllname.ax .
regsvr32.exe dllname.ax

This must be run at an elevated command prompt so that UAC cannot stop the registry access that's required. Ask more questions about this at superuser.com

If the DLL is 32 bit:

1. Copy the DLL to C:\Windows\SysWoW64\
2. In elevated cmd: %windir%\SysWoW64\regsvr32.exe %windir%\SysWoW64\namedll.dll

if the DLL is 64 bit:

1. Copy the DLL to C:\Windows\System32\
2. In elevated cmd: %windir%\System32\regsvr32.exe %windir%\System32\namedll.dll

Finally I found the solution just run CMD as administrator then write

cd \windows\syswow64

then write this

regsvr32 c:\filename.dll

I hope that answer will help you

On a x64 system, system32 is for 64 bit and syswow64 is for 32 bit (not the other way around as stated in another answer). WOW (Windows on Windows) is the 32 bit subsystem that runs under the 64 bit subsystem).

It's a mess in naming terms, and serves only to confuse, but that's the way it is.

Again ...

syswow64 is 32 bit, NOT 64 bit.

system32 is 64 bit, NOT 32 bit.

There is a regsrv32 in each of these directories. One is 64 bit, and the other is 32 bit. It is the same deal with odbcad32 and et al. (If you want to see 32-bit ODBC drivers which won't show up with the default odbcad32 in system32 which is 64-bit.)

Open the start menu and type cmd into the search box Hold Ctrl + Shift and press Enter

This runs the Command Prompt in Administrator mode.

Now type: regsvr32 MyComobject.dll

If the DLL is 32 bit:

Copy the DLL to C:\Windows\SysWoW64\
In an elevated command prompt: %windir%\SysWoW64\regsvr32.exe %windir%\SysWoW64\namedll.dll

if the DLL is 64 bit:

Copy the DLL to C:\Windows\System32\
In an elevated command prompt: %windir%\System32\regsvr32.exe %windir%\System32\namedll.dll

I know it seems the wrong way round, but that's the way it works. See:

http://support.microsoft.com/kb/249873
Quote: "Note On a 64-bit version of a Windows operating system, there are two versions of the Regsv32.exe file:
The 64-bit version is %systemroot%\System32\regsvr32.exe.
The 32-bit version is %systemroot%\SysWoW64\regsvr32.exe.
"

I just tested this extremely simple method and it works perfectly--but I use the built-in Administrator account, so I don't have to jump through hoops for elevated privileges.

The following batch file relieves the user of the need to move files in/out of system folders. It also leaves it up to Windows to apply the proper version of Regsvr32.

INSTRUCTIONS:

• In the folder that contains the library (-.dll or -.ax) file you wish to register, open a new text file and paste in ONE of the routines below :

  echo BEGIN DRAG-AND-DROP %n1 REGISTRAR FOR 64-BIT SYSTEMS
  copy %1 C:\Windows\System32
  regsvr32 "%nx1"
  echo END BATCH FILE
  pause
  

  echo BEGIN DRAG-AND-DROP %n1 REGISTRAR FOR 32-BIT SYSTEMS
  copy %1 C:\Windows\SysWOW64
  regsvr32 "%nx1"
  echo END BATCH FILE
  pause
  

• Save your new text file as a batch (-.bat) file; then simply drag-and-drop your -.dll or -.ax file on top of the batch file.

• If UAC doesn't give you the opportunity to run the batch file as an Administrator, you may need to manually elevate privileges (instructions are for Windows 7):

  1. Right-click on the batch file;
  2. Select Create shortcut;
  3. Right-click on the shortcut;
  4. Select Properties;
  5. Click the Compatibility tab;
  6. Check the box labeled Run this program as administrator;
  7. Drag-and-drop your -.dll or -.ax file on top of the new shortcut instead of the batch file.

That's it. I chose COPY instead of MOVE to prevent the failure of any UAC-related follow-up attempt(s). Successful registration should be followed by deletion of the original library (-.dll or -.ax) file.

Don't worry about copies made to the system folder (C:\Windows\System32 or C:\Windows\SysWOW64) by previous passes--they will be overwritten every time you run the batch file.

Unless you ran the wrong batch file, in which case you will probably want to delete the copy made to the wrong system folder (C:\Windows\System32 or C:\Windows\SysWOW64) before running the proper batch file, ...or...

• Help Windows choose the right library file to register by fully-qualifying its directory location.

  1. From the right batch file copy the system folder path
     • If 64-bit: C:\Windows\System32
     • If 32-bit: C:\Windows\SysWOW64
  2. Paste it on the next line so that it precedes %nx1
     • If 64-bit: regsvr32 "C:\Windows\System32\%nx1"
     • If 32-bit: regsvr32 "C:\Windows\SysWOW64\%nx1"
       • Paste path inside quotation marks
       • Insert backslash to separate %nx1 from system folder path
     • or ...
       
       

• Run this shotgun batch file, which will (in order):

  1. Perform cleanup of aborted registration processes
     • Reverse any registration process completed by your library file;
     • Delete any copies of your library file that have been saved to either system folder;
     • Pause to allow you to terminate the batch file at this point (and run another if you would like).
  2. Attempt 64-Bit Installation on your library file
     • Copy your library file to C:\Windows\System32;
     • Register your library file as a 64-bit process;
     • Pause to allow you to terminate the batch file at this point.
  3. Undo 64-Bit Installation
     • Reverse any registration of your library file as a 64-bit process;
     • Delete your library file from C:\Windows\System32;
     • Pause to allow you to terminate the batch file at this point (and run another if you would like).
  4. Attempt 32-Bit Installation on your library file
     • Copy your library file to C:\Windows\SystemWOW64
     • Register your library file as a 32-bit process;
     • Pause to allow you to terminate the batch file at this point.
  5. Delete original, unregistered copy of library file

There is a difference in Windows 7. Logging on as Administrator does not give the same rights as when running a program as Administrator.

Go to Start - All Programs - Accesories. Right click on the Command window and select "Run as administrator" Now register the dll normally via : regsrvr32 xxx.dll

And while doing this, if you get error code 0x80040201, try the solution in DllRegisterServer failed with the error code 0x80040201, but make sure, you open command prompt as Run as Administrator.

Everything here was failing as wrong path. Then I remembered a trick from the old Win95 days. Open the program folder where the .dll resides, open C:/Windows/System32 scroll down to regsvr32 and drag and drop the dll from the program folder onto rgsrver32. Boom,done.

Part of the confusion regarding regsvr32 is that on 64-bit windows the name and path have not changed, but it now registers 64-bit DLLs. The 32-bit regsvr32 exists in SysWOW64, a name that appears to represent 64-bit applications. However the WOW64 in the name refers to Windows on Windows 64, or more explicity Windows 32-bit on Windows 64-bit. When you think of it this way the name makes sense even though it is confusing in this context.

I cannot find my original source on an MSDN blog but it is referenced in this Wikipedia article http://en.wikipedia.org/wiki/WoW64

You need run the cmd.exe in c:\windows\system32\ by administrator

Commands: For unregistration *.dll files

regsvr32.exe /u C:\folder\folder\name.dll

For registration *.dll files

regsvr32.exe C:\folder\folder\name.dll

Here is how I fixed this issue on a Win7 x64 machine:

1 - error message:

"CoCreateInstance() failed Plkease check your registry entries CLSID{F088EA74-2E87-11D3-B1F3-00C0F03C37D3} and make sure you are logged in as an administrator"

2 - fix procedure:

• Start/type cmd/RightMouseClick on cmd.exe and choose to "Run as Administrator"
• typed: regsvr32 /s C:\Program Files\Autodesk\3ds Max Design 2015\atl.dll regsvr32 /s C:\Program Files\Autodesk\3ds Max Design 2015\MAXComponents.dll
• restart Win 7 and back in business again !

Hope this helps !