1) Try Safe Mode First
When your PC just won’t boot into Windows at all, you’ll probably want to start off with Safe Mode—use the F8 key right when the system starts up to access the menu and see if Windows will boot into Safe Mode. If you’re able to get into Windows, the problem is likely something that would load only when you boot normally—graphics card drivers, junk applications, or perhaps some spy ware applications.
If Safe Mode Doesn’t Work
If you can’t even boot into Safe Mode, you should use the Startup Repair tools, which can be accessed from the Advanced options in the Safe Mode menu; you can pull out your system repair disc, or your Windows installation disc and use the Repair option. If you’re having a bootloader problem that says “bootmgr is missing”, you can use the command prompt from an installation CD and use the bootrec /fixboot command.
2) Use System Restore
System Restore in Windows XP might not have been the greatest tool, but since Windows Vista came around, it’s been an excellent choice for fixing up all sorts of problems, including issues that cause your system to not boot anymore. While testing, I once deleted a bunch of files in system32 to purposely cause a blue screen of death, and then used System Restore to get the system up and running again in minutes. If you’re unfamiliar with the process, you can follow this step-by-step guide to restoring your PC with System Restore.
3) Try Booting Off an Ubuntu CD
Whenever I’m having issues with a PC not booting up properly, one of the first things I do is use a free Ubuntu Live CD and try and boot from that to see how well the system is working. If everything is working in Linux, then you know the problem is likely just your Windows installation, and not hardware related. This has saved me a lot of time in my DIY PC repair.