What OS?
If you download Antivir from their site and not the mirrors, the definitions will be from that day. You have to click on the Antivir symbol on the download page.
Antivir will install and run in safe mode. So getting the install file on the system somehow, like via a cd that you burned on another computer, and then running the install in safe mode, to prevent the possibility that whatever is on the system will block anti virus programs, should be a good start.
If you where going to make a cd, throw Ad Aware and the latest def.ref, Spybot S&D and the latest "includes" update, and Hijack this on it. They will also install and run in safe mode.