Fake antivirus? I get those. That is a worm. All you need is the internet open, and it can get in, it doesn't need to be attached to anything in particular to get in. It is one that frustrates me to no end, because I can never get rid of it and just end up leaving my laptop off for a week or so. It does dissappear if you do that though, because teh person trying to get into your computer gives up XD
My source? After getting it on 3 different computers and my laptop at least twice, I did some serious research on how to stop it and where it was coming from.