Conficker, the Downadup worm affecting 3.5 million PCs so far.

1 in 3 Windows PCs vulnerable to worm attack

The Downadup worm, called "Conficker" by some researchers, surged dramatically this week and has infected an estimated 3.5 million PCs so far, according to Finnish security company F-Secure Corp. The worm exploits a bug in the Windows Server service used in Windows 2000, XP, Vista, Server 2003 and Server 2008.

Microsoft issued a patch in late October after confirming reports of in-the-wild attacks, most of them against machines in Asia.
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Microsoft has recommended that Windows users install the October update, then run the January edition of the Malicious Software Removal Tool to clean up compromised computers.

When you see the little yellow shield, apply the updates. If you don't see the little yellow shield for over a month, you probably have something infecting your machine, blocking the updates. Update your anti-virus program and run a full system scan. I recommended this anti-virus/anti-spyware program: AVG Free 8.