I use 98 (se) an havent had this problem.What other AV do you have installed?.Im not sure what to advise here if as you say you have all windows critical updates installed , and you have thouroughly uninstalled and reinstalled avast.One last thought may be to run system file checker.Type sfc.exe in start>run and scan for altered files.Maybe that particular file will show up and prompt you to replace the file from cab files.A rule of thumb (if you havent used system file checker before ,is if it finds that a file has changed..possibly because its been replaced by a more up to date one... keep the newer one.You will be able to tel this by the version numbers)
Other than that maybe repairing IE will do the trick.Go to add/remove programmes and clcik remove and it will offer to take you back to the original version of IE.Of course you will have to perform windows update again to get it back to IE6 and fully patched.Other than that im not sure what to recommend.
me