WINXP PRO SP2 SYSTEM SHUTDOWN BLUE SCREEN

Hello, completely new to this forum. Came here becuase I have a home built computer and am having blue screen lock ups and system reboots during DEFRAG, system virus scans, and anything that requires the HDD to be accessed for reasons such as I mentioned above to include webroot scanning. I have built this system which has been stable up unitil now. I will list my config below. But here is what happened. I noticed after about two weeks after I returned from Afghanistan and installed the Norton and WEBROOT I was fine. After about twoo weeks later I noticed when I was scanning that it locked up then did a reboot. Shortly there after norton scaned and did the same thing. Then when I did a defrag it did the same thing. I uninstalled Webroot to narrow it down and no dice. Also removed all recent windows updates to narrow that down. Not sure what the deal is so I put a new Mboard as a upgrade with new ram and new video card and same thing still happens. I bielve its something related to windows updates but I have uninstalled SP3 and it still messes it up.

Mboard MSI P7N Platinum SLI LGA 775 ( Latest Chipset Driver installed from NVIDIA )
Proc C2D E6600 65W @ 2400MHz (not overclocked)
Video Card EVGA GTX 280 1Gig Ram (not clocked)
Ram GEIL Evo One (4 Gigs) DDR2 (not clocked) PC26600
WINXP Pro SP2 (legit Copy)
HDD Samsung 500Gig SATA 3gig
PS 700W OCZ