You mention that you have not installed any new hardware. I had a similar locking up/freeze/crashing problem with XP Pro.
Eventually traced the problem back to a USB device.I had a printer installed via a self powered USB 4 port hub. The computer was giving problems,and I found this was when I used the printer, more often than not. I was unable to solve the problem, so I contacted the manufacturers. Their advice was to use a straight USB port connection on the computer, and not use the self powered hub. Tried it and it, and everything as been okay since.Not saying this is the answer to your problem, but worth checking your USB connections,and devices attached,as a try out.