6-15-10 UPDATE: Microsoft Security Essentials is our current AV of choice for catching this - See Step 6 below.
About every six months a really bad virus comes down the pike that gets around the best Anti-Virus protection.
Some of the variations of this are made to look like a Windows Security feature, so even if you think it's legit, assume it's not. Some variations also open a new browser window with a "My computer" box and makes you think it is scanning your system.
The key to killing this virus is immediate action as soon as you see the box pop-up.