Wow, this is a blast from the past — a worm that overwrites the Master Boot Record (MBR) of the hard drive. I haven’t seen one of those since the early nineties. But believe it or not, that’s not really destructive… just fdisk /mbr to rebuild the MBR, and unless the worm has done a lot more to your system, you’re back in business.
I think fdisk is XP-specific and earlier, they’ve replaced it with diskpart or something in Vista/7.
Hm, you’re right, it’s not there now, apparently even in XP. But there’s a new utility specifically to fix the master boot record: Fixmbr. It’s “only available when you are using the Recovery Console,” but it should do the same thing.