New to using the command prompt? Check out the Windows Command Prompt Beginner’s Guide
If you’ve ever asked a PC help question in a forum, it’s likely someone suggested you check for corrupted or out of version protected system files. To do this:
- Open a Command Prompt (Click Start, type cmd, and click cmd.exe.)
- Type sfc /scannow and press Enter
What Does the sfc /scannow Command Do?
System File Checker (SFC) verifies all protected system files are current and complete. If they’re not, it replaces them to bring them into currency.
Here’s what Microsoft say (applies to Windows XP):
System File Checker gives an administrator the ability to scan all protected files to verify their versions. If System File Checker 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. System File Checker 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. 
*The DLL cache folder is replaced in Windows Vista and 7 with Windows File Protection (files are kept in C:\Windows\winsxs\Backup.) Thank you Marc Liron, Microsoft MVP, for this information.
This is a good thing. If something doesn’t seem right on your PC, run this command to give you peace of mind and a good start.