I had a fairly modern IBM ThinkCentre with XP Pro on it.
I let the SP3 updates load and BOOM!
It will not reboot, no matter what I do.
I had to get an an older computer from a friend to use.
hmmm... Boom? Did you blue screen? I’ve heard of some trouble with SP3, but I didn’t experience any difficulty.
Can you boot up in Safe Mode? (hit F8 before the Windows welcome screen attempts to load - while you’re still seeing Black and white DOS loads.) I just looked in the Msft knowledge base and download area
Microsoft has a SP3 patch that may solve your problems.
Search microsoft.com/downloads for this Update for XP KB953979