Scott has noted that if you are updating XP and have selected IE8 as one of the downloads, you will most likely get a run.dll error after you reboot and you will still have IE6/IE7 on your system.
You have to do yet another update and reinstall IE 8 again and this should resolve the error.
This does not happen if you run IE8 from a standalone install, only through Windows Updates.
Hope This Helps
Scott