Windows XP SP3 More Harm Than Help
[...] Within hours of its release, Microsoft’s Service Pack 3 for Windows XP began drawing hundreds of complaints from users who claim the update is wreaking havoc on their PCs.
Looking elsewhere, it seems the continuous reboots are caused by MS loading special code for both AMD and Intel CPUs, regardless of which CPU you have.
Always a good idea to not to be the first kid on the block with new software updates...