What do you do when your computer won't even boot up as far as windows? There are various errors that you could possibly see during your computers initial boot up procedure. By this I mean, when all the text is scrolling by on a black background, before you see the initial Windows start up screen.
USB hard drives (backup drives) are the worst culprits for these errors. It can read "Unable to read from disk", "Invalid hard drive", "Boot disk failure" or something similar, leading you to believe that your beloved hard drive has just given up the ghost. The problem I was having in this case was a blank screen with the cursor blinking in the top corner. This can result from the computer trying to read your back up drive as if its the main boot drive, but since there is no actual copy of your operating system (Windows) on it, it throws out errors like there's no tomorrow. Luckily, its easy to diagnose. Simply turn off your computer, unplug the USB cord from your computer, and restart. If you boot successfully, it was your USB drive. Now that you've figured out why it wouldn't boot up, what now? Well, particularly in older versions of windows, you won't be able to power up your computer with the usb drive plugged in. Its a pain, but you'll have to wait until your computer is completely booted up, and then plug it in. There are options you can try to change in your BIOS, but can be risky if you don't know what you're doing, and is more advanced than I want to get in this posting. Do a google search for USB BIOS settings if you feel capable of changing your bios settings. In this case, the USB drive was completely toast, and had to be replaced.
The same principle applies to any USB device if you are having boot up problems. Unplug ALL of your USB devices, and try to boot up the computer. If you get a successful boot up, then shut it down, plug in ONE USB item (printer, scanner, camera etc) and try to boot again. Keep going until you find the peripheral that is causing the boot problem. Once you've found it, update its drivers, check the manufacturers website, post on message forums, email me, or do whatever you have to do to get that pesky item setup properly again.