[Update (April 3, 2009): Apparently, not much happened on April 1 with this virus. Many of the commenters below said this virus was a hoax, or at least not a big deal, but after poking around the web I would disagree. At best, opinions appear to be mixed on it; some people are saying it's nothing, others are saying it could have been bad. The virus itself is real enough, and the links below have more info if you're curious.]
This is not a joke: there is a worm getting spread on PCs called Conficker.C that will be activated on April 1 (no, this is not a joke) and may cause huge disruptions in the 'net. My brother, who is a computer techie, has more on his blog, and he notes you can download a free removal tool at EnigmaSoftware.
Making sure your anti-virus software is up-to-date may not be enough, so if you use a PC be very careful with this! Don't screw around; go to the above link right now, grab the tool, and check for this malware. I ran it on the two PCs in our house and we're fine. Are you?