My computer won't allow me access to any programs from safari to games to iTunes. What do I do?

Whenever I click on something a little window pops up and tells me that the program is infected with "W2/Blaster.worm" and it keeps trying to get me to buy "Malware Protection" for $60. I assume that's part of the virus but I don't know what to do. I use McAfee and it's scans aren't coming up with anything. A little window keeps popping up saying "Security Warning! Malicious program has been detected. Click here to protect your computer." And that just comes up with Malware Protection saying how infected my computer is. I don't know what to do. Please help ASAP.