Under Outlook when you delete your messages from the Inbox they are automatically moved to the Deleted Items folder. If you don't manage this folder regularly, it will definitely increase in size and may even slow down the startup of Outlook Express.
The solution is to configure Outlook to automatically clean out this folder.
In Outlook Express, open the Tools menu and click Options.
The "Other" Tab.
Under the General tab, select the Empty messages from the Deleted Items folder upon exiting.
Now, the Deleted Items folder is automatically emptied when you exit Outlook.