This is a common - almost unanswerable - problem. As you've both reinstalled XP my mind is moving towards a hardware failure/conflict/corruption.
This thread is old but has several tips, some of which work for the users some don't. However, towards the end, two posts refer to hardware fixes.
http://www.computing.net/security/ww...rum/14619.html
Hope this helps. Either way please post back to let us know how you get on.
OJ