Presumably you are using OE6.
Try this fix:

Start > Search > For File or folders > All Files and Folders.

Type in inbox.dbx

When the file is located look in the right hand pane and right click the file and select "Open Containing Folder".

Cut and Paste all the files from that folder into a new folder.Use any name for the new folder,but remember where it is.

When you next open OE your mail folders will be empty. If this solves the problem you can try importing the mail from the folder you moved them to.

Go to File > Import > Messages. Select Outlook Express 6 and click next. Put a dot next to "import from an OE6 store directory" and click next, navigate to the folder that you moved the files to,click next and import the mail.

Hope that helps

Thanks to The Globe and Mail