Looks like there is a limited problem with SP3 causing endless reboots among HP computers with AMD processors (also possible: mobo issue). They’re working on it as we speak, and the root of the problem appears to be a power management driver that is only capable of causing trouble in this exact situation. It’s like the appendix: no apparent function except to blow up and kill you.
I can’t imagine any of you are having this problem if you’re reading this page, but if you know someone who does, my advice is to stroke their hair softly and tell them everything’s going to be all right.