Too bad Macs are awfully overpriced. But they are vunerable to viruses as well.

Anywho, you been hit by malware and it sounds like WinAntiVirus, WinFixer or whatever. I'm not much of a tech junkie. My brother was in a similar situation a while back and I can't recall what malware remover was used to get rid of it. There are spyware removers you can use or you can attempt to remove it manually.

If you can, try these links:

http://www.pcindanger.com/winantivirus-pro-removal.html
http://www.spywareremove.com/removeWinAntiVirusPro2007.html
http://www.xp-vista.com/spyware-removal/windows-antivirus-pro

Edit:

I remember what I used. It was Malwarebytes:

http://www.malwarebytes.org/