With April 1st around the corner everyone seems to be on the look out for the Conficker worm. Here is some good info.
Quick overview of the worm from Wikipedia:
“Conficker, also known as Downup, Downadup and Kido, is a computer worm that surfaced in October 2008 and targets the Microsoft Windows operating system. The worm exploits a known vulnerability in the Windows Server service used by Windows 2000, Windows XP, Windows Vista, Windows Server 2003, Windows Server 2008, and Windows 7 Beta. The latest variant will begin checking for a payload to download on April 1, 2009.
The Conficker worm spreads itself primarily through a buffer overflow vulnerability in the Server Service on Windows computers. The worm uses a specially crafted RPC request to execute code on the target computer.