A friend of mine got hit with a virus a couple of weeks ago and that key in the registry that deals with how to handle .exe files was modified the way RIP! mentioned... for that particular virus, the instructions from Symantec included this:
Start the computer in Safe mode, then
Click Start>Run type cmd
If you aren't already in your Windows folder (the folder name before your flashing cursor... eg. C:\Windows) you'll need to go there. To do that, type "cd \" (without the quotes)hit enter, then type "cd windows" (again, no quotes) and hit enter again.
Type "ren regedit.exe regedit.com" ( -quotes) and hit enter. Now type "regedit" and the registry editor *should* open and you can do whatever editing you need to do.
This should work for any virus/trojan that modifies how Windows handles .exe files, but if it also modified how .com files are handled, this won't help.
Hopefully this information will be useless and no one here will get another virus.