Vista has decided it's pirated. Help?

The Vista machine I was having problems with is now updated to SP2... Unfortunately, as soon as the update finished applying, a popup window came up claiming that an unauthorized change has been made to Windows, and the operating system must be reinstalled.

Is there a way to fix this, or am I going to have to break out the system disk?

Edit: also, I'd like to avoid System Restore if possible, because I don't want to have to do the SP2 upgrade again (and risk the same thing happening, again).

Edit 2: MGADiag says the license number is invalid. Does this have any bearing on what I can do?