Windows stops working unless you keep task manager open.

I have a client that is running XP Pro and it is also a satellite for QuickBooks Pro 2009. The problem is that the computer will freeze up and nothing will work. You can move the mouse but it won?t click on anything. If you hit ctrl-alt-delete and open task manager you can then navigate for a bit. Then it will freeze again after a couple of minutes and you have to 3 finger salute again even though task manager is already open. This is the second time this has happened at this place. The first time the computer was old so it was replaced with this one. I have done several scans for viruses and malware and spyware. It comes up clean in all instances. I have Specter Pro on the computer as well and I can track down to the second when the computer stops functioning but there is nothing strange or funny that happens. She was using QuickBooks and was on a secure website that they use everyday. I am at a loss why this is happening. Also if I close all running processes to QB in task manager it will unfreeze it for a few minutes then you have to ctrl?alt-delete again no matter what. Any ideas?