At the office we have a few Vista 64 machines and no lockups. But there is no damage to the OS, no registry cleaners and they get their yearly cleanings (canned air blasts, etc.)

I disagree that it's the OS in this case. But if you think XP will fix it, what's stopping you?

You ask how to fix it but offer little clue to what may be the cause. It sounds like a hardware or heat issue. But what have you done to remove that as a possible cause?
Bob