There has been a lot of buzz about the MBR rootkit, spotted in the wild recently. This rootkit requires administrative privileges, thus presenting yet another reason to log into Windows as a standard user. If you have Windows Vista, be sure you have the UAC enabled.
See the Antimalware Team report on the MBR rootkit for further details regarding this rootkit. If you suspect your computer has been infected, contact Microsoft Customer Support Services at no charge using the PC Safety line at 1-866-PCSAFETY (North America) for assistance. For information on international locations for assistance go to http://support.microsoft.com/security.