Someone (anonymous) has actually beaten me to this: Vista doesn't run the virus, but the virus does run Vista. Still a victory for backward compatibility though in my opinion!

Re: @Leo, in fairness, Vista couldn't be expected to block the exection of a boot virus. That would be like expecting the US Army to do something about the Roman Empire, in more ways than one; Vista is nowhere in sight while the virus does its business, and the virus has come and gone by the time Vista boots.

it's not easy for AV software to quarantine or destroy something that's overwritten your boot sector; there might even be a bit of touchy legality involved in copying the real thing back. Cleaning the virus out is one thing, but doing so in such a way that your PC is still capable of booting is not trivial since the virus is now your bootloader...