main suspects are the hard drive or the PSU (specifically the PSU fan).
Test the hard drive with drive maker's diagnostic software which loads from a bootable CD (you cannot test the Windows system drive with Windows-based software). The links are here: http://www.tacktech.com/display.cfm?ttid=287
The download is usually an 'ISO' file from which you can create a CD using IMGBurn from here: http://www.filehippo.com/download_imgburn/
Boot your PC from that CD to test the drive.
In the case of the PSU, it can't be tested. Just replace it if it's suspect. Take the opportunity to fit one with a higher wattage rating (output), especially if the existing one is the original factory-fitted unit.