Ram, Overheating, Bad SATA/IDE controller driver, Bad Video Card Driver.
1) Download the latest reference (manufacturer) driver for your video card.
2) Download the latest reference (manufacturer) driver for your IDE/SATA controllers - may be from your motherboard manufacturer.
3) Install the reference drivers.
4) In windows devices manager (right click my computer ->Properties->Hardware->Device Manager-> IDE ATA/ATAPI Controllers & also SCSI devices) delete your primary Dual channel PCI IDE controllers, or nvidia, or whatever. Don't bother deleting your Primary/secondary channels, they're just part of the main controller). Reboot.
Windows will automatically reinstall the latest drivers (which you just installed earlier) when it reboots.