Elite Ian

My Desktop PC keeps freezing after various periods of use from 5 minutes up to one hour. After it has frozen it then has problems re-booting where it freezes at the bios splash screen. I have to remove all cd / dvd's from the drives to get it to re-boot. The spec is; Asus 3.4 ghz processor 1.5gb ram 512mb Graphics Card (Previously 128mb and had same problems then) 320 gb Hard Drive. Running Windows 7 Ultimate. I have replaced the graphics card and the hard drive but still have the same problems. Even tried formatting the hard drive and installing Windows XP but for some reason it wont allow me to do this. Any ideas (other than take a hammer to it) would be very welcome? Thank you.