System File Checker
(Very) Brief History
Due to problems with certain programs/malicious software over-writing system files in Windows 95 and Windows 98, Microsoft implemented some security to protect from corrupting Windows installs. While the release of Windows ME started the protection system, it wasn't really that effective until the release of Windows XP and the introduction of Windows File Protection and the System File Checker. Using the System File Checker Utility, the user can scan for and correct any errors in Windows system files.
Usage
By running sfc /scannow the utility will proceed to scan the system files. If it finds a problem, it can correct it by loading files from the dll cache (%SystemRoot%\WINDOWS\System32\Dllcache\, where %SystemRoot% is the drive windows is running from, usually C:). This folder is hidden, to unhide it in Windows Explorer, go to Tools->Folder Options, View Tab, select Show hidden files and folders, and uncheck Hide protected operating system files (Recommended). If a dll file stored in the dll cache becomes corrupt as well, you will need your Windows XP Install CD in the CD drive. To avoid using the installation disk, you can copy the 470MB+ i386 folder to %SystemRoot%\i386, and again, %SystemRoot% is usually your C: Drive.
For more options, see the links.
Note: This utility is also available on Windows Server 2003.
Also, please note that this may override Hotfixes. [1]