I'm assuming you are fairly tech-illiterate, so i wont ask too much of you.
You can try start the computer in Safemode (turn computer on and press f8 until you get the option for safe mode. use your arrow keys and highlight safemode, and then hit enter).
log onto whatever is the administrator account and see if you can run your anti virus there. If you can't, then you will find it very very tough to remove.
I would suggest removing the hard drive, and attaching it to another computer (set autoplay to disabled) and scan it using that pc) but that's a bit more technical than i'm guessing you're able to do.
Failing that, find a techy person who can do the job properly for you, and doesn't charge a lot