When I download files that contain viruses mostly from Limewire how do they infect my computer when I don't even execute them? :-?

I understand that they would show up in my virus scans just for being there, but how do they execute themselves without me starting them? I ask this because I thought I was reasonably safe scanning all the files I downloaded before I ran them, but I downloaded a virus and prompty deleted it after discovering it, but it kept DUMPING me to the BSOD after a few minutes of starting Windows.

The problem has since been fixed, but I am curious, and yes I know Limewire and other P2P networks are infested with this sort of stuff.There's some code that makes it run immediately. I know, not very helpful, but if we posted how to create the basis for viruses, they'd pop up in great numbers... WRAITH.......
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but I downloaded a virus and prompty deleted it after discovering it, but it kept dumping me to the BSOD after a few minutes of starting Windows. The problem has since been fixed, but I am curious, I know Limewire and other P2P networks are invested with this sort of stuff.