It sounds as though you email files are corrupt.
I would do the following to create new ones....
Go to C:\documents and settings\username\local settings\application data\identities\?????? and cut/paste the outlook express folder to another section of your hard drive, it doesnt matter where. Now open outlook express and it will create a new empty set of files.
Go to file/import/messages/outlook express 6 and then select the bottom option to browse. Browse to the outlook express folder (not inside it) whereever you pasted it. Now you can import your emails into the new files. Any corrupt files wont import but you will at least be able to use your inbox again.