Windows XP has the ability to protect itself from system instability caused by 3rd party software overwriting important system files. This used to be (and still is in fact), a problem with Windows 95 and Windows 98.
With the introduction of Windows Millennium Edition, Microsoft made a real effort to stop this from happening.
Now in Windows XP we have a much more refined protection of these important files....

This system is called: Windows File Protection

By default, Windows File Protection is always enabled and allows Windows
digitally signed files to replace existing files safely. Currently, signed files are distributed
through:

# Windows Service Packs
# Hotfix distributions
# Operating system upgrades
# Windows Update
# Windows Device Manager
If you introduce a file replacement in any other way, Windows File protection will overwrite your file!
An important part of Windows File Protection is the command line utility:

System File Checker (sfc.exe)

You will often see references to scannow sfc in online newsgroups etc. This is a great tool for troubleshooting Windows XP problems.
How to use scannow sfc...

http://www.updatexp.com/scannow-sfc.html