(NB: This method deletes all mail in your 'Sent Items' folder. If there's something you want to keep, create a new folder and move any important mail items to there before following these instructions):
If you're using Outlook Express, close the program and run a search for *.dbx.
(You might need to ensure that 'show hidden files' is checked)
That will find all of the files which make up the various folders of Outlook Express. Right-click on 'Sent Items' and select 'Delete'. Restart your PC and launch Outlook Express. It automatically looks for the standard folders (such as 'Sent Items') and, if it can't find one, creates a new (empty) folder with that name. (i.e. you'll magically discover that you've now got a fully functioning 'Sent Items' folder, which is empty).
If you're using Outlook (rather than Outlook Express), the above method will still work but you need to search for *.pst, instead of *.dbx.
If you're using another email client, post again telling us what it is) and we'll try to find a solution.
Chris