Hello.
Your system file is corrupted.
You could try to repair, if you have the Windows CDs:
http://support.microsoft.com/kb/314058 See Option 1 and Option 2
If still a problem, try System Restore: Boot your system to Safe Mode (press F8 repeatedly). Go to System Restore and choose a date when your system worked and follow the prompts.
If still a problem, try Factory Restore, which would take your system to when you purchased it initially. Boot your system and press F11 repeatedly. Follow the prompts.
Hope one of these choices work. If not, you may need to reinstall the operating system from scratch.