There's no need to reinstall Outlook Express. The problem lies with an error in the Inbox file. Simply deleting the Inbox, and then allowing Outlook Express to create a new Inbox (which the program will do automatically), will fix the problem.
Close Outlook Express
Click on Start > Search > For Files and Folders > All Files and Folders
Type 'inbox.dbx' in the search field.
Click on More Advanced Options
Put a tick next to 'Search Hidden Files and Folders'
Click Search
When the file is found, right-click on it and select 'Delete'
Click 'Yes'
Open Outlook Express, which will now have a brand new (empty) Inbox.
Please note that following those instructions will delete all mail in the old Inbox. If it's possible to do so, any important mail should be transferred to other folders first.
Chris