Worst Virus Ever

I work on peoples computers part time to get some extra money on the side. Last week I encountered a disgusting virus (mmfav.dll as i recall) living in the system32 folder.
Heres where it gets tricky. Everytime the antivirus software or anti spyware software scanned for it, it shut down not only the program but the entire computer. So i tried to go in manually. I went into System32, targeted the file and the computer shut down. I went back in, tried to just right click on it, worked, then tried to delete it. I didn't expect it to work, and it didn't. (currently in use, as mr.gates told me).
So I tried just to target it with the anti-virus softwares manual virus vault. and the damn thing just wouldn't budge. Eventually I just fixed all the ways it was f**king up the computer, (i.e. accessing certian folders, and ip's) but it is still on their, probably doing bad stuff.
Someone suggested KILLBOX to me, thoughts?