In windows 7, if some of your system file is corrupted , then some Windows functions may not work, or Windows may crash. You can follow the below steps to let System File Checker tool (SFC) scans for missing or corrupted system files and repairs them.
1.Go to Start — All Programs –Accessories ,right-click Command Prompt, and then click Run as administrator. If you are prompted by UAC for an administrator password or for a confirmation, type the password, or click Allow.
2.Type in command sfc /scannow then press ENTER .Then The sfc /scannow command scans all protected system files and replaces incorrect versions with correct Microsoft versions.
3.If the SFC command can not repair some file,please go to location of this file and Take Ownership ,then replace the file with a known good copy of the file
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http://mabdelhamid.wordpress.com/2011/11/27/how-to-repair-windows-system-files-with-system-file-checker-sfc-2/