The System File Checker scans the integrity of all the protected system files. If it finds a incorrect or modified system file, it replaces it with the correct version.
the following is from: mvps.org
“To check your Windows protected files, proceed as follows:
1. Click the Start button
2. From the Start Menu, Click All programs followed by Accessories
3. In the Accessories menu, Right Click on the Command Prompt option
4. From the drop down menu that appears, Click on the ‘Run as administrator’ option
5. If you have the User Account Control (UAC) enabled you will be asked for authorisation prior to the command prompt opening. You may simply need to press the Continue button if you are the administrator or insert the administrator password etc.
6. In the Command Prompt window, type: sfc /scannow and then press Enter
7. A message will appear stating that ‘the system scan will begin’
8. Be patient because the scan may take some time
9. If any files require replacing SFC will replace them. You may be asked to insert your Vista DVD for this process to continue
10. If everything is okay you should, after the scan, see the following message “Windows resource protection did not find any integrity violations”
11. After the scan has completed, Close the command prompt window”