System files can sometimes get written over or deleted. SFC (System File Checker) scans your Windows XP system and replaces the missing files. Missing system files can cause startup/boot problems or other operating system headaches. SFC is part of the WFP (Windows File Protection) feature that prevents programs from writing over critical system files.
I was once working on a computer that was having a startup problem, ran the command line utility SFC /scannow, and voila! Problem fixed. SFC scans the system and determines if there are any missing files. If so, the files are obtained from cache (%systemroot%\system32\dllcache). If not in cache, SFC will obtain the files from the installation source on the network or the Windows CD-ROM. You might be asked to insert the Windows Installation CD-ROM.
To use the SFC utility: