There is a new worm out called "Conficker" (also known as "Downadup" and "Kido") that has reportedly infected millions of Windows PCs. It has been detected by various antivirus companies and unfortunately it is suspected to become active tomorrow (April 1st, 2009) so currently there is little known about what it will do.
Fortunately for most Mac users, this worm is Windows-only, so there shouldnt be any problems. However, many Mac owners have windows or windows-compatible environments running on thier systems in the forms of Virtual Machines (Parallels and VMware) as well as direct installations via Bootcamp. In addition, there are several windows emulation environments such as Codeweaver’s Crossover which will run windows programs directly on your mac, and may allow the worm to execute.
Given the concern about this threat, we strongly recommend you install and run antivirus software on your windows installations. In addition, while this threat does not yet affect OS X, you may consider running a virus scanning package on your mac as well to prevent spreading the malware to Windows users. Be sure to also fully update your Windows installations, since Microsoft has recently released patches to help tackle this threat.
More information on this virus can be read at the following locations: