If you have not installed any other IE5/OE combination you can go to control panel/add-remove programs. Second tab is the Windows setup, one of the choices is outlook express. Uncheck the box, click apply, click okay. Find file msoe.dll, delete file, reboot system, go back to same tab in Add-remove programs, then re-check box, apply, okay.
If you have installed any updated IE5/OE versions, you remove them in the first add/remove dialog box. Find the file msoe.dll and delete, reboot system, do a custom reinstall IE5/OE ( this will allow you to make sure that the Outlook Express tab is checked, and any other things you want to add that doesn't get installed by default install ).