Mix & match here 
Downstairs main PC is running XP.
Upstairs secondary PC is running Vista Home Premium.
and - I have Vista Ultimate ready and waiting from my MSDN subscription. The problem is that being in IT my main PC is so loaded with stuff that I know it is going to take 3 full days + to reinstall everything (if I'm lucky). Plus I'm not confident that it is all Vista compatible, in fact I know a proportion of it isn't.
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Lets be careful out there !