My PC Won't Boot
When Keatlaretse boots his PC, it shuts itself down. He's thinking about reinstalling Windows, but doesn't want to lose all his data.
Obviously, your first priority is getting that data back. Recovering your PC comes second.
Of course, if you backed up your data regularly like you should, that wouldn't be an issue. See What's the Best Way to Backup What I Need to Backup? for ideas on how you can build better habits before your next disaster.
Okay, on to your problem:
On another computer, download the live version of Puppy Linux and burn it to a CD-ROM. It's an .iso file, and double-clicking the file will likely bring up your burner. If it doesn't, download and install ISO Recorder. Puppy isn't the best Linux flavor by a long shot, but it's small, fast, and is easy for Windows users.
Boot it on another computer first, just to make sure the disc is okay. Then boot it on your broken PC.
If your PC won't boot to CD any more than it will boot to Windows, you've got a hardware problem. If you're not comfortable diagnosing and fixing computers, bring yours into a repair shop. But make sure the people in the store understand that your first priority is rescuing data off of the hard drive. They'll charge you extra for that chore, but that's the price of not backing up.
But if you can boot into Linux, copy as much as you can to an external hard drive. Then you can try to fix Windows.
Remove the CD and boot the PC, pressing F8 just before the Windows log-on appears (it may take a few attempts to get the timing right). At the Boot Menu (assuming you get that far), select Last Known Good Configuration to restore your Registry to an earlier, happier time.
If that doesn't work, reboot and press F8 again, but this time select Safe Mode. Once booted, select Start, All Programs, Accessories, System Tools, System Restore. Follow the wizard's instructions and pick an appropriate backup.
What if you can't even get to the Boot Menu? Or you can, but system restore doesn't fix the problem? You'll have to use the diagnostic and repair tools that Microsoft put on the Windows XP CD and Vista DVD. That is, you can if you have one of these discs; if Windows came with your computer, you probably don't.
Luckily, there are free, alternative, downloadable versions: the XP Recovery Console CD and the Vista Recovery Disc. These are both .iso files (like Puppy Linux) that you can burn to bootable CDs. When you boot them, they look and behave exactly like the XP and Vista installation discs, except that when you attempt to install Windows, you get an error message. Only their repair tools work.
If your PC won't boot XP, boot from a real XP CD or the Recovery Console CD. At the main screen, press r for Repair to launch the Recovery Console. This DOS-like, command-prompt environment offers several useful repair tools, including:
bootcfg: View and repair the boot.ini file.
diskpart: manages partitions (warning: it destroys them).
Fixboot: Rewrites a partition's boot sector.
Fixmbr: Fixes the master boot record.
If your PC fails to run Vista, boot from a real Vista DVD or the Vista Recovery Disc. Click the Repair your computer button and then Startup Repair. Everything is pretty simple from there.
If nothing else works, you may have to resort to reinstalling Windows. See Reinstall and Restore Your Windows PC in Eight Easy Steps for instructions.