I think there was an issue with the new prescott processors and SP2, that SP2 wouldn't boot if the processor was present, and microsoft fixed it with an update. Don't ask me how you can update your windows if you can't boot into it, but that's microsoft for ya!
Here, I found the article, it's at
http://support.microsoft.com/default...b;en-us;885626 but if this is not the case for you then I dunno, sorry