Something has corrupted the boot information on your hard drive so Windows can't load. This might mean the hard drive itself has failed. but it might also just be a software problem. If you can borrow a Windows installation CD, you can try repairing the software. You didn't give your operating system, so I'm assuming you have XP.
On every Windows XP CD (it doesn't matter which version, Home or Professional, full or upgrade), there is a very handy tool called the Windows Recovery Console. Here is how to try repairing your Windows installation using it:
- Put your XP install CD in the drive and restart the computer so it boots from the CD. If your computer isn't set to boot from the CD first, you will need to change the boot order. Watch the screen when the computer starts up. On some systems you can reach a boot menu by pressing the specified key, or you might need to change the boot order by going into BIOS setup. Again, the key to press will be given at startup.
- After some time loading files from the CD, the setup program will show a screen saying "Welcome to Setup". Press R to start the Recovery Console.
- The Recovery Console will list Windows installations it finds and ask which you want to log on to. Usually there is only one installation, so press 1, then ENTER. (Note: At this point, if there are no Windows installations listed or you've gotten other error messages, either your hard drive is bad or your Windows installation has been wiped out, possibly due to a virus. All you can do is try a fresh, full reinstallation of Windows.)
- You'll be asked for the Administrator password. Unless you changed it when Windows was working, the password will be blank, so press ENTER. You will see a C:\WINDOWS> prompt.
- Enter the command CHKDSK /P and press ENTER. (You don't need to enter the command in capitals, I just did that for readability.)
- Let the command do its work. You'll probably see several messages about recovering or repairing errors. When it's finished, the Windows prompt will return.
- Type EXIT and press ENTER. Your computer will restart.
- Remove the CD so Windows will try starting from the hard drive normally. If the repair was successful, whatever was corrupted will be fixed and Windows will start up properly.
If the repair doesn't work, again it's either hardware (you need a new hard drive, or in some rare cases a new system board) or the Windows software was too damaged for repair and needs to be completely reinstalled.
Good luck with your repair, and thanks for using Fixya!