2 new catalyst computers locking up

We have bought 2 new computers, Both from M3 Computers. They lockup after about 10 - 30 minutes. We had them loaded with XP home for the simple reason i hate vista LOL. We took them back to the shop and they said that they had it on all night and that they did a bunch of tests and all went fine. I picked them up and took them back to the office and about 10 minutes after i fired them up, they both locked up. They are identical systems and i am not sure of the specs but i do know that they have belkin wireless cards in them. the M3 folkd seem to think it is an electrical problem but i dont really see that being the issue seeing as that the office was just completed about 2 weeks ago. I checked the voltage in the outlets and installed surge protectors on the outlets that the computers are plugged into. anybody got any ideas what the problem could be?