System File Checker scans all protected files to verify their versions. If it discovers that a protected file has been overwritten, it retrieves the correct version of the file from the cache folder
(%Systemroot%\System32\Dllcache) or the Windows installation source files, and then replaces the incorrect file. It also checks and repopulates the cache folder. You must be logged on as an administrator or as a member of the Administrators group to run System File Checker. If the cache folder becomes damaged or unusable, you can use the sfc /scannow, the sfc /scanonce, or the sfc /scanboot commands to repair its contents. In that case you will be prompted to use the windows installation disk to recover lost or corrupted files
This is how you run SFC on your computer
( for xp - Click on Start > Run Type cmd press ok The black command window opens ) ( for Vista - Open an elevated command prompt. To do this, click Start, click All Programs, click Accessories, right-click Command Prompt, and then click Run as administrator. If you are prompted for an administrator password or for a confirmation, type the password, or click Allow. )
Type in front of the command prompt > sfc /scannow and press Enter (There is a space between sfc and /)
You may be asked to insert windows cd in the drive bay. Do so to continue. Rest is automatically done.
Reference xp ms kb 310747 for vista mskb kb 929833applies to all versions of windows Xp and 2003 and Vista In previous version of windows system file checker is available as a menu option in system tools > system information
