This might have something to do with your Deleted Items folder becoming corrupted. Compacting the folder should solve this.
Click on the Delete Items folder. Click on File --> Folder --> Compact
Once this process is complete, restart Outlook Express, and see if the delete button begins working again.
For a corrupted Deleted Items folder:
- Find out which version of Outlook Express you're using by clicking Help --> About Outlook Express
- Close Outlook Express.
- For Outlook Express 4: Locate the files Deleted Items.mbx and Deleted Items.idx in the mail directory and delete them. You can use Start --> Find --> Files or Folders --> "Deleted Items.??x" to find them.
- OE5+: Locate the file Deleted Items.dbx in the mail directory and delete it. You can use Start --> Search --> Files or Folders --> "Deleted Items.dbx" to find it. Since you're on Windows XP, you need to use the advanced options to find Hidden or System files.
- Open Outlook Express again. It will create a new empty Deleted Items.
See if your Delete key works at that time. If it does not, unfortunately, it looks like your Inbox may have become corrupted. You will need to uninstall and reinstall Outlook Express.