Are the exact same programs running on both computers?

No, I don't think it's a problem with Windows XP. It could well be a conflict with an add-in from a program you have on one computer and not the other. Go to Tools/Templates and Add-ins on both to see what else may be loading with Word. You can move add-ins out of Word's Startup folder and see if that makes a difference. Close Word first, then use Windows Explorer to move the file(s).