Remove Critical Alert from Microsoft Fake Alerts

Please help me. I keep getting a "Critical Alert from Microsoft" informing me of a virus infecting my computer and threatening to disable it if I don't contact Microsoft etc... The Error code is (268D3 - XC0037. Has anybody else had this problem and if so, how do I get rid of it???? This problem keeps occurring every day now and is driving me bonkers. McAfee does not seem to pick this up, therefore won't activate