How to check and repair corrupt system files in Windows Vista Home

1. Boot your computer into Safe Mode, especially if you can't access the regular Windows Vista desktop due to a missing system file. Shut off your computer and wait a few seconds, then turn it back on and tap 'F8' before the Windows splash screen loads to access the Advanced Startup Menu. Use the arrow keys to choose 'Safe Mode' and press 'Enter.' This loads a driver-sparse version of Windows that is useful for troubleshooting.
2. Click 'Start' and choose 'All Programs' and then 'Accessories.' Right-click on 'Command prompt' and choose 'Run as Administrator.' Click 'Allow' if Windows prompts you to confirm that you are the computer administrator. The command prompt opens: this is a DOS-like interface where you can enter commands to run utilities for troubleshooting and repair.
3. Type 'sfc /scannow' at the prompt (the flashing cursor) and press 'Enter.' This starts Windows System File Checker, which scans the Windows installation directory for any corrupt, missing or non-authenticated versions of the Vista files. By default, the program replaces the files that don't match with System File Checker's record of your Vista install. When dealing with a virus this is particularly helpful, since some viruses replace system files with lookalikes that have the same name but destructive purposes.
4. Type 'Exit' when the System File Checker completes it diagnostic and press 'Enter' to return to Safe Mode. Shut down the computer normally -- by using the 'Start' button -- and then restart it and let it load to Vista regularly.