@Baxter's is mostly correct but it is missing one important Windows-specific detail.
Subversion's runtime configuration area is stored in the %APPDATA%\Subversion\ directory. The files are config and servers.
However, in addition to text-based configuration files, Subversion clients can use Windows Registry to store the client settings. It makes it possible to modify the settings with PowerShell in a convenient manner, and also distribute these settings to user workstations in Active Directory environment via AD Group Policy. See SVNBook | Configuration and the Windows Registry (you can find examples and a sample *.reg file there).
In TortoiseSVN you can access and modify some settings in the context menu "Settings/General" in explorer.
E.g. the "Global ignore pattern" can be edited directly in the dialog box. Such changings are the stored in the registry as bahrep pointed out in her answer.