Boot time virus scan?

I am working on a machine that locks up as soon as the desktop is loaded, sometimes before the icons even pop up. Safe mode works fine. I've disabled all services possible and it still happens. There are no available system restores to go back to. AVG is loaded on the machine but the only way to use that in safe mode is through the command line interface. When I try to run it it says the registration code is invalid. I can't install something else because windows installer will not load in safe mode. The person I am working on this for has limewire, bearshare, and kazaa loaded on it as well as vuze, utorrent, and frostwire. They are also dumb as a rock when it comes to computers so I am assuming they got some kind of virus that is killing explorer.

What would you guys recommend as a boot time scan for virus' in this situation?

tl;dr Family has screwed up the machine and I need a bootable antivirus to scan.