I had this happen once in such an amusing way - it prevented me reinstalling XP. Couldn't even use the XP cd to format and install. Had to use a linux CD to wipe all MBR info and then reinstall XP.

(I know, M$ is bad, but I have to have it on one machine for the games which I can't get on Linux yet)
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Rory
MkIIa, blue lit buttons, memory upgrade, 1Tb in Subaru Forester STi
MkII, 240Gb in Mark Lord dock
MkII, 80Gb SSD in dock