It's an increasingly common trick. Your PC is infected by a virus, which gives fake alert warnings that you've been infected by some non-existent malware. And if you fall for it then you're directed to buy and install fake antivirus software to remove the "threat", even though it never existed in the first place.
Your existing antivirus software should catch these "FakeAlert" viruses, but if it doesn't (or you don't have any antivirus protection) then this free McAfee tool may be able to help. There's nothing to install, just download and run the program (right-click and run it as an Administrator in Vista and Windows 7), point it at the drives you'd like to scan, and the program will check for malware and remove any specimens that it finds.
If FakeAlert Stinger doesn't find anything the first, then you can try increasing its heuristic sensitivity (click Preferences and set Sensitivity Level to Very High). This may also increase the chance of false alarms, though, so we'd recommend you set the "On virus detection" setting (in the same dialog) to "Report only".
The threats removed by McAfee Labs FakeAlert Stinger are extensive. Refer to the Mcfee Labs website for more information.