I installed Diffuse in Windows Tortoise CVS and it could not have been easier.
All you have to do is open TortoiseCVS Preferences, click Tools tab, and browse to diffuse.exe under "Diff application"
under Diffuse where you installed it. I changed the install path to C:\Program Files\TortoiseCVS\Diffuse.
Thats it. Thanks for making the interop with Tortoise so easy.