That’s a rather risky thing to do. If you don’t know what you’re doing you could really mess things up.
But I’m going to assume you’re not a newbie and you know what you’re doing.
If you’re using Windows 9x or Me, go to the system.ini file and look in the [driver32] and [driver] sections. Anything that has vidc.fourcc=codecfile.ext associates the codec file with a given fourcc. Delete and add as needed. The [driver] section handles 16-bit codecs, and the [driver32] section handles 32-bit codecs.
In Windows 2000 and XP you have to run the registry editor. Go to start, run, then type regedit. Then navigate to the key HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows NT/CurrentVersion/Drivers32. Notice the string values with vidc.fourcc and their associated codec files? Good. Now you can add, remove, and force associations as you wish. If you want to get fancy, you can go into the key right above it, HKEY_LOCAL_MACHINE/SOFTWARE/Microsoft/Windows NT/CurrentVersion/drivers.desc and add the name of the codec file and a custom name for it so it appears readable on the codec list.
Note: Some codecs will have the option to enable or disable which fourcc’s they handle - the main ones I can think of are xvid, divx 5, and 3ivx. They often offer (well, actually take over is more accurate) to handle other flavors of mpeg-4 (ms-mpeg-4, divx 3, divx 5, xvid, and 3ivx)