I would start by uninstalling anything that isn't essential in running the computer out (sound card, NIC, etc). See if the problem exists without any of these. Then one by one reinstall them and their drivers. Make sure any devices that are onboard (sound, NIC) are disabled in the BIOS after you uninstall them in Windows. I had this problem on an old computer on win2000. It turned out to be a firewire card I had installed.