Hi BAP --
I almost hesitate to say anything because it's only based on v6, not v7 (FAH version that is, not to be confused with Windows 7). BUT, with that disclaimer, the file you're after that needs deleting is called client.cfg in FAH6, perhaps it's the same in 7. HAVING SAID THAT, in FAH6 there was a client.cfg file for both cpu and gpu folding, just in different places (or directories, or folders).
If you're clear on the disclaimer above, go ahead and do a File Search and maybe both will come up, one in a cpu or smp-related direcory and one in a gpu-related directory. If you've done this and have got this far, delete the gpu-related one and a new install should allow you to proceed.
Let us know, it's always nice to see you and anyone else bringing on new firepower and the team's been doing not all too shabby as of late as it is!
P.S. Remove or Delete Programs (in Windows Control Panel would have been another way to do it, at least in Win XP.