[stonehenge]

It's Vista or W7 right? I did some gewglin and people have various answers:

• Some say you might be able to fix it by redoing the boot record. You'll have to boot to a repair disc and run these from the command prompt:
  
  c:
  bootrec /fixmbr
  bootrec /fixboot

• Rootkit. Try ComboFix.

• Use the TDSSkiller from Kapersky to remove this MBR-virus

There were some other "answers" too but the above are fairly easy to try and shouldn't screw anything up if you already have an ok boot record and no rootkit.

Edit: Like cao said, one of those "other answers" was to replace the ataport.sys file with a known good one. To do that you'll have to use some sort of boot/repair disc because I don't think Windows will let you swap that file while it's running.