If Windows XP (or 2000) refuses to start, press F8 right after you turn on your PC but before the Windows log-on appears (it may take a few attempts to get the timing right). At the resulting menu, select Last Known Good Configuration to restore your Registry to an earlier date.
If this doesn't get your PC working, reboot and press F8 again, but this time select Safe Mode, and then choose Start ► All Programs ► Accessories ► System Tools ► System Restore. Follow the Wizard's instructions and pick an appropriate backup.
If that approach doesn't work either, or if you can't even get to this menu, use your emergency boot floppy. If your hard drive's boot sector or Windows' basic boot files have been corrupted, this disk will circumvent the problem and boot you into Windows. If you don't have an emergency boot floppy, you may be able to use one created on another PC running Windows XP or Windows 2000, but there's no guarantee that it will boot your machine.
To make one, insert a blank floppy disk into drive A:, select Start► All Programs ► Accessories ► Command Prompt. Type "format a:" and press enter. When asked if you want to format another disk, type "n" and press enter. Type the following commands, pressing enter after each one:
xcopy c:boot.ini a: /h xcopy c:ntdetect.com a: /h xcopy c:ntldr a: /h
Now type exit and press enter to close the window. Remove the floppy disk and label it "Windows XP boot floppy." Put this emergency disk in the floppy drive of your inoperable machine and boot up. Windows should run with no problems. You could simply keep the floppy in the drive all the time, but to truly fix the problem, launch the command prompt as described above, type "xcopy a:*.* c: /h", and press enter.
If the emergency boot floppy doesn't work, try the Recovery Console, a Windows utility that provides a DOS-like command line from which you can run some repair programs. It's tricky to use if you're not accustomed to command lines, and you can damage your data, so be careful.
If you have a Microsoft Windows CD-ROM, you can get to the Recovery Console by booting from that CD and pressing any key when you're told to 'Press any key to boot from CD'. At the 'Welcome to Setup' screen, press "r" for Repair.
Please let us know how useful you find this tip by rating it below! If you have a useful Windows tip, timesaver or workaround to share, submit it to our tip contest and you could win a prize!