With Windows 95 Microsoft learned that much of the instability of Windows was caused by 3rd party applications overwriting Windows system files and causing havoc. To counter this Microsoft released the System File Checker(SFC) in Windows 98. With this utility one can scan all Windows System Files and restore altered files.
How to scan for altered files or restore a single file using SFC.EXE
Note that one can set the SFC to point to a different directory to restore
files from. This can be useful in case one have upgraded to a newer version of the Internet Explorer,
where one should point SFC to the install files of the newer version. If restoring Internet Explorer
related files from the original CAB-files will lead to mixed binaries with unpredictable results to follow.
Note that Windows Me uses System File Protection
and doesn't have a System File Checker.
More Info MS KB185836
More Info MS KB188186
More Info MS KB192832