Windows XP users -- you may receive a message that begins with:

Window Update has determined that you are running a beta version of Windows XP Service Pack 1 (SP1). To help improve the stability of your computer, Microsoft recommends that you remove the beta version of Windows XP SP1 and re-install Windows XP SP1. If you cannot remove the beta version, you should still reinstall Windows XP SP1.

The message contains an attachment named "Winxp_sp1.exe," which if downloaded (as the message urges users to do) will cause your system to be infected with a Trojan horse program, TROJ_XOMBE.A. So if you receive this message, just delete it. Be sure not forward it to anyone else.