XP Machine slow, Freezes Intermittently

I have a 2-year old homebuilt system - GA 965P-S3 iP965 motherboard, Intel Core 2 Duo E6400 2.1 GHz, 2x 1GB Corsair XMS2, DDR2 PC5400 (667) RAM, Radeon X1650 display adapter, 320 Gb Seagate Barracuda 7200.10, NEC Labelflash DVD. OS is dual-boot XP SP 3 and Ubuntu 8.10.

All worked fine until recently, when it became slow, and would freeze for a few moments then continue under XP. I erased the HDD, and reinstalled windows which improved, but didn't solve the problem. I've tried disconnecting the DVD, removing DIMMs and swapping DIMMs into different slots, none of which have helped. When I move the mouse around the pointer periodically stops momentarily, then restarts. I've tried a different mouse, and get the same problem.

The problem occurs only very rarely under Linux.