Try this from Start > Run:
sfc /scannow
This runs a validation check against your current copies of files, to the installation media (note: you need your original, or a copy of your original OS disks).
If you dont have that option, then try to reinstall IE. To do this edit this registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Active Setup\Installed Components\{89820200-ECBD-11cf-8B85-00AA005B4383}\IsInstalled and change it to a 0. Try and run Windows Update and it should work.
If this isnt possible then upgrade to IE7. That should replace the file (tbh, I think this is the easiest option).