Do some hardware tests to see what's causing the problem:

Hard drive testing software: http://www.tacktech.com/display.cfm?ttid=287

RAM testing software: http://www.memtest.org/

Both require that you make a bootable CD from an ISO file. Use IMGBurn on a working PC to do that: http://www.filehippo.com/download_imgburn/

Boot your PC from each CD in turn to run the tests.
For hard drive test select the long/extended test.
For RAM test, test each module separately for at least 3 passes on each module.