Try BartPE first - you create the CD then boot into a basic type of windows where you can rename the current registry files and then copy a set from a previous restore point. It might be enough to get you back into Windows to either fix it or do a repair install or whatever:

http://www.nu2.nu/pebuilder/

I had loads of problems getting .net 2.0 off my machine to install something else, the setup kept bombing out and undoing the uninstall, can't remember how I fixed it but I recall it not being pretty!

Gareth