can’t open outlook express

Hi,

Ever since I have had to install an older version of IE for an application to work, IE 5 to be exact, I have been unable to open Outlook Express even though I have migrated back to IE 7. Here is the Error message I get: “Outlook express could not be started because MSOERES.DLL could not be found. OE may not be installed properly.” I have tried uninstalling using the ADD/Remove in the control panel, I have tried using the REG EDIT to remove it as being active, I have reinstalled IE 7 many times. I have tried to replace the files contained in the Outlook Express folder with those on the Program CD. The only thing I have not done is reinstall windows XP entirely.
Please help I have been 2 months without this working.

Thanks