Assuming that you're using XP, have you tried a repair?
Sometimes the system files (those required to run Windows properly) can become corrupted. Here's how to run a check on your system files:
• Click Start; Run.
• In the Run box, type cmd.exe to open the command prompt window.
• At the prompt, type sfc.exe /scannow
• Press ENTER. If Windows finds problems with the system files, you will be prompted to insert your Windows XP installation CD to restore the files.
If no problems are found and the scan finishes, type exit and press ENTER.