Did you get a message like this?
BOOT SECTOR WRITE
If so, this is displayed by the BIOS code of your machine. The
text is displayed almost always when you try to write to the
master boot sector of your machine.
This does not necessarily mean you have a virus. You could be using
FDISK, Format or disinfecting a virus. If necessary, turn the warning
of from your BIOS settings, look for an option like "Virus protection".
If you get the message when booting with a floppy in drive A: or when
running an unknown program, it is probably caused by a real virus.
[Analysis: Mikko Hypponen, F-Secure]