Lost File Association With Exe Files

The aftermath of a trojan attack (Vista Guardian) has resulted in the file association of exe files being lost and hence an inability to run exe files. I am quite convinced that the trojan is now dead, but I can't figure out how to fix the damage. If I could get into the registry I could fix it, but of course regedit is an exe file so I can't run it. I've tried tricks like running it from command line or from inside a batch file but with no joy. I have also made a copy and called it regedit.com. This looked romising as Vista asked for user permission to run the program, but then nothing. It gets killed off before it starts. I am not familiar with the particular machine, nor am I very familiar with Vista and I am now at a loss as to how to proceed.