TortoiseSVN installs the TSVNCache.exe file which runs in the background. It displays an icon overlay indicating the SVN status of a file. When a file is written to disk, TSVNCache checks to see if the folder containing the file contains a .svn folder.
If TSVNCache performs a lot of disk activity, you can select the folders that it listens on by going to TortoiseSVN -- Settings -- Icon Overlays, excluding the root of all your disk drives (make sure you add an "*" at the end), and including the folders containing your source code. You may need to restart for the change to take effect.
Alternatively, you can set the Status cache to None if you don't want the icons - this will turn off TSVNCache entirely.
You can use Filemon from SysInternal to check your mileage.
To get a tray icon for the cache, you can set the HKEY_CURRENT_USER\TortoiseSVN\CacheTrayIcon DWORD key to value 1.