Time has not been kind to IE. The leak of Microsoft's source code found exploits to IE versions 5.5 and prior which is now creating no end of problems for users that think "it worked for years" and are faced with new issues. Some won't upgrade, so its best to let them ponder and explore why the world has changed.

On top of that is the PARASITE issue (reference http://www.doxdesk.com/parasite/ ) which frankly is over 1/2 the machines I deal with, a basic cause of the users complaints.

To tackle your issue, I might try the Add/Remove software tool and see if the Add/Remove of Internet Explorer has a Repair option.

After that, your post may have a typo in it. Your title and writing notes "Internet Explorer", but the error message is not that, but "Explorer." These are two programs.

"Explorer has caused an error in MSHTML.DLL" will not be fixed in the same way as "Iexplorer has caused an error in MSHTML.DLL"

http://www.google.com/search?&q=%22Explorer+has+caused+an+error+in+MSHTML.DLL%22
http://www.google.com/search?&q=%22Iexplorer+has+caused+an+error+in+MSHTML.DLL%22

Yields different avenues.

Bob