I'm assuming this is an XP/2k/Nt forum issue, so I'm going to move you there.
Are you able to boot through to Windows with that error?
Have you tried going to start>run and entering:
Or doing a File Search for it. You should have a c:\I386 directory which is a cabinet archive. Is it there.
If you cannot boot the system, you will need to follow the instructions for restoring it from the CD....
If you've verified that the file is actually missing from that location and you have no other source for it, you can get it here:
... but if you can't boot the system and have a NTFS file system, I'm not sure how you would restore it that way. You will need to restore it from the CD or cabinet files.