To stop it rebooting, go to Control Panel>System, then click "Advanced System settings" in the left-hand pane. In the System Properties dialog, click Settings under "Startup and Recovery", and then uncheck "Automatically restart". After that it will give a Blue Screen error which is easier to troubleshoot.
Then download UBCD and burn it to CD, it has all the memory testing and disk diagnostic utilities on it plus many more besides. Run MemTest and Windows Memory Diagnostic, as well as the disk utility for your brand of hard drive (or Seatools if you aren't sure). All you have to do is to boot up with the disc in the drive, select the utility that you want to use, and it will run.
Ultimate Boot CD