[Mister X]

It looks like this is actually a worm of some sort. It's called the MiMail virus it seems. here's some stuff about it from MSNBC:

“I WOULD LIKE to inform you about important information regarding your email address. This e-mail address will be expiring. Please read attachment for details,” claims the e-mail, which has been dubbed “MiMail” by antivirus companies. It is particularly believable because it appears to come from the local network’s system administrator. Copies received by MSNBC.com, for example, came from “admin@msnbc.com.”
If a recipient clicks on the attached file, the virus inserts a small program on the victim’s machines that scans all files to compile a list of e-mail addresses, according to Network Associates Inc. These are then stored in a file called “eml.tmp” on the victim’s computer, perhaps for later retrieval.
Network Associates spokesman Vincent Gullotto said the virus may have gotten its initial kick-start because it was initially sent out in a spam mailing.
MiMail spread quickly. Trend Micro said about it had cataloged about 1,000 infections from U.S. customers, including some Fortune 100 customers by Friday afternoon.
“That’s pretty good for the opening hours of the game,” said David Perry, Trend Micro spokesman.
E-mail lists devoted to computer security were abuzz with reports of the virus.
William Shepherd, AntiVirus Administrator at the International Truck and Engine Corporation, said his office was getting slammed by copies of the virus.
“I received a notice from Trend (an antivirus firm) about 1 1/2 hours ago and I’ve already had to block 50 e-mails,” he said.
But Vincent Weafer, senior research director for Symantec Corp., said he was optimistic the virus outbreak would fade away quickly after its initial flare-up.
“I don’t think it’s going to go very far,” he said. Spam-initiated computer viruses often end up as “one-shot deals,” he said.

Looks like it might not be too serious but don't take any chances. Update your antivirus software and get any updates available at windowsupdate.com. My Norton detected the virus after I updated it.