This occasionally happens after a power failure, improper shut-down or attack by a virus programme. But that doesn't mean your system is gone. Try the following simple steps.
1. Boot from the windows CD from which you installed your operating system.
2. Press R to go into recovery console.
3. Press 1 or the other type of installation if not 1.
4. Type the administrator password then press ENTER.
5. Type MAP then press ENTER.
6. Type "copy(drive letter where the CD is):\i386\ntdlr c:\" then press ENTER.
7. Type "copy(drive letter where the CD is)ntdetect.com c:\" then press ENTER.
8. If prompted for the boot.ini file, create one using notepad and copy it to the WINDOWS
folder like above. i.e., "copy(drive letter where the file is):\boot.ini c:\ then press ENTER.
TIP: you can get the boot.ini file from another computer, it's located in the WINDOWS folder.
I hope it works for you, good luck.