The banner itself appears to advertise and link to a download called Free PC-Secure (which is unfamiliar to us) but the real problem is that its appearance automatically launches a Windows dialog box telling you that "Your system is not clean" and asking you if "you want to download System Doctor to improve it." You know, the typical hoax message that tries to download a malicious ActiveX control even though you click "Cancel" in a fruitless attempt to ignore it.  The conclusion here is that a nasty spyware ad seems to have infiltrated into Microsoft's banner advertising network because one of their clients is making use of some html code trickery.

We advise everyone to pay strict attention when they receive such a message window out of the blue. Cancel any initiated downloads and close the pop-ups/browser instances it launches. The animated banner in the screenshot below (showing two random frames) is the cause of the unsolicited Windows prompt. The bottom image shows the deceitful message on the page that is automatically opened no matter your choice in the dialog window.

Update: Microsoft has admitted to the mistake and pledged to improve its advertiser screening process. But it's a major blow for the company which has been devoting years of effort to convincing users Windows is now a safer OS.