If you’re a Windows user System File Checker is a really useful tool in your armoury . It repairs broken or missing Windows system files and often fixes obscure problems or improves Windows performance.
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.