You must overwrite the file located in the \"File Protection Service\" folder, which is C:WINNTSYSTEM32DLLCACHE. This folder is normally hidden from prying eyes, but Windows will reveal it to you if you ask it nicely...
Make sure you replace the MSPAINT.EXE in that folder before any other copies of the file you may have.
On a separate note, have you tried running MSPAINT from another folder, like your desktop? If you still get the same error...maybe your problem isn\'t MSPAINT at all....:-)