Property Mogul, you need to disable all the email accounts that are configured in Outlook.
By disabling all the email accounts, Outlook will not try and send anything from the Outbox, as Outlook has nowhere to send it to. So you're able to start Outlook and delete the email
For XP, click Start-Control Panel-Mail-Email Accounts-View/Change and disable the accounts ( can't tell you the exact steps right now, all I can see are Exchange-connected w/s. If you are really stuck post again and I'll fire up a POP3 w/s to give you a step by step guide )
Once you've stopped all email accounts from being processed, start Outlook and delete the offending Outbox file, then exit Outlook, go back into Control Panel - Mail, and re-enable the mail accounts.
richard