This is why I mentioned that it could be a possible hardware problem being an older system. I had one old stubborn board here that saw memtest report all kinds of crazy errors on two totally different sets of memory. Once on a new board both sets where found 100% error/defect free!
That old board seemed to love eating batteries seeing all kinds of crashes, unexpected system restarts and even pointing a cpu problem. After sitting apart for over a year the case was reassembled with a new battery to suddenly see everything working like brand new?!
HP's troubleshooting section is more pdf files then anything as seen at http://h20000.www2.hp.com/bizsupport...iesId=470559#8
A hidden hardware problem would account for seeing persistant problems even after reformatting the drive and performing clean installs of Windows. The hard drive seeing bad sectors, the onboard Intel graphics if you haven't installed a separate card, bios needing update, and some other things would be showing signs of wear on the older model.