hi akassahun - there is some debate on this topic, but i'm not convinced that once you have been infected by a trojan that you'll ever really know that you don't have some residual threat on your system. i'd be interested in the opinions of others, including the good people that run the SYMC labs - i'll refer to US-CERT as my reference on this. i think all AV/threat products depend on being on the system prior to infection and that removal from a machine that was unprotected at the time of the infection would require expertise most users don't have. you could use all kinds of tools to monitor process and connections, running down common and exotic load points and maybe get it that way, but would ever really know you go it?
anyway, sorry for the detour, but you brought up an excellent question i haven't seen here previously.
question on the problem you're having - what happens when you try running regedit, msconfig or other utilities. specifically, are you having problems running .exe's in general? if so, you might try renaming regedit.exe to regedit.com and see if that runs.
mel
