System File Checker is a very useful tool in the armoury of Windows users. It repairs broken or missing Windows system files, it often fixes obscure problems which defy resolution, and it usually improves Windows performance to a greater of lesser degree.
In Windows 7 or Windows Vista run the command sfc /scannow from an “elevated” Command Prompt. For a more detailed description of System File Checker, elevated Windows Command Prompts and how to use them see this mistywindow page.
You really need System File Checker.