bad virus, need help

my computer has been infected witha virus called TROJAN.WORM.variant, all these anit-virus ads wont stop poping up and my comps slowed down. my McAfee virus scan isnt picking it up and my parents are mad at me because i downloaded the file that had it. how do i get rid of it without having to buy something? is there a way for McAfee to get it?

Ads are usually caused by spyware, not viruses. Download Ad-Aware or Spybot Search and Destroy.

AVAST. It caught so many things off my computer... It hasn't missed anything that the other name brands miss. Plus, it's free.

thanks for the advice but McAfee finnally caught it on the 3rd scan