If you have successfully sent the email then you can then delete it from your outbox,this shouldn't pose any problem, then just make a full scan of your system to see if there is any virus present

Here is what I did to quickly fix this problem.
Close out Outlook.
Using Explorer on your local hard disk, locate your message store under Local Settings/Identities.
Most likely you will have to change your folder view to enable viewing all hidden folders and filenames.
After you do this find your message store.
If you don't know how to find this go back to outlook under Tools, Options and Maintenance to find out where Outlook stores your message store files. Now close out Outlook.
Once you get into this folder sort by filename and find the file called Outbox.dbx.
Change the name of this file to something else, like xOutbox.dbx.
The next time you open up Outlook, Outlook will create a new Outbox.dbx and your problem is fixed.

By the way...
If you want to have a new Sent.dbx, rename your Sent.dbx to something like xSent.dbx and Outlook will recreate a new Sent.dbx completely empty. All your Sent messages are not lost they are just stored in your old renamed file called xSent.dbx. You can always retore it later if you want, but do remember you now have a new file called Sent.dbx; first rename it to something like zSent.dbx or some naming convention of your own creation.