If they are in your Sent folder, they're already Sent (or more accurately
Outlook connected to a server, told it to send them, and the server said,
"OK, thanks."). Mail that's in your Outbox is mail that's still waiting to
be sent.
I don't know what version of Outlook you're running, but if you go to
http://support.microsoft.com/kb/195922 and look at the "Create a New
Personal Folders File" section you can see how to create a new PST file and
move your mail to that and see if your problem persists (I've used these
steps up to Outlook 2003).
If you cannot delete them this could indicate a corrupt PST file (assuming
you're not using Exchange Server, in which case you'd need to contact your
mail administrator).