Heat or power supply problems can cause such issues.. Virus and spyware scanners stress the machine somewhat and can make them work a little more than normal...therefore causing machine shut down.

First, in order to make those scans a little easier for the scanner programs, make sure to defrag the hard drive and run Error Checking/Chkdsk to fix any problems that could be causing the scans to hang up. Clean out some of the unnecessary junk files and eliminate unnecessary startup programs as well.

How To Run 'Error Checking/Chkdsk' On Win2000/XP

After that, to check for heat issues, please take the cover off the tower, then blow out all dust and junk from inside, around the fans, power supply, CPU, and heatsink, using compressed air, then point a house fan at the innards and run the scans while it's set like that. If the scans run fine now, then you know the culprit. (While inside the computer, check the motherboard capacitors to see if any are leaking or bulged.. Bad capacitors can cause the same type of issue.)

If the steps above don't fix the problem, the next step will be to replace your power supply with something with a higher number of watts.. Most brand name computers have power supplies that need improvement.. Install one with 450 watts or so and you should see a change.

Hope this helps.