Could be that your OE index file is corrupted.
Use search to locate "folders.dbx" (on advanced search options turn on hidden and system files) once found right click and rename folders.dbx to folders.old.
Re start OE (this rebuilds the index file)
Hope this helps.
I have problems in sending out my emails using outlook express, however, i still able to receive my emails after installing WinXP.
I have check with my ISP, the settings are alright.