My ASUS (quad core) Windows 7 (64 bit) machine just won't boot. It has 1.75 TB hard disk space: one 1 TB Western Digital HD, and a defective (stock) Seagate 750 GB HD (unplugged and disconnected because of defect). My first guesses about the possible cause(s): 1. Bad hard drive: No, because the BIOS recognizes the hard drive and Windows files can be read. It just won't boot. 2. Bad memory: after two passes, the Windows Memory Diagnostic Tool didn't find any memory errors. after the test, it automatically reboots, but it just won't boot. 3. incompatible driver(s): No, because it won't even boot with the original installation disc or boot disc. It just won't boot. Besides, I haven't installed anything (hardware or software) recently. I was working on a Word document when it crashed. The stupid machine just won't boot. 4. bad video or video driver: No, because I tried to boot in Safe Mode with command prompt but I got the same problem. The logo displayed, so it is possible to display graphics. Once I even got into Safe Mode and it looked like Windows (GUI), not like DOS. But when I selected Recovery, it went back to reboot, and it just won't boot. 5. Something I don't understand: probably 6. Bad motherboard: maybe 7. Your idea. . . After several unsuccessful attempts to start in safe mode and safe mode with command prompt, I changed the default boot drive to the DVD drive and placed, alternatively, the installation disc and a boot disc created with an image of a working boot disc. Pressing F8 or F9 during boot-up, I got various boot failure messages. Here are two of them: "If problems continue, disable or remove any newly installed hardware or software. Disable BIOS memory options such as caching or shadowing. If you need to use Safe Mode to remove or disable components, restart your computer, press F8 to select Advanced Startup Options, and then select Safe Mode. Techical information: *** STOP: 0x0000003B (0x00000000C0000005,0xFFFFF8000D002F8C,0xFFFFF88005135E50,x0000000000000000)" What happens is, it starts to load and it says "Loading Windows" and the Windows logo appears pulsating. Then the "no signal" message appears from the HDTV I'm using as a monitor, then the power supply fan and/or the hard drive kicks into high gear (I'm listening to the sound--I think it's the fan), and then it starts the POST test again. This loop continues no matter what I do--even when I try to boot from the hard drive, or from a boot disc, or from the original install disc. Upon another attempt to load, I got this error: "Windows failed to start. . ." and other words too. It had a full page of words, but the following are important: "\windows\system32\boot\winload.exe" and "0x0000001"