A nasty little virus that installs in the MBR and steals banking login
details is starting to make it's way around. Because it's in the MBR,
anti-virus software can only alert you to it's presence, but can't remove
it.

http://news.bbc.co.uk/2/hi/technology/7183008.stm

According to Symantec, running "fixmbr" command from within the Windows
Recovery Console will remove the virus, but to prevent infection, if your
BIOS has a MBR write-protection feature now is a good time to enable it.