To find the Secure Temp folder, you have to hop into the registry.
Where the number is the number of your version of Outlook. 15 is Office 2013. The registy entry will give you the file path of the Secure Temp folder, go to it and delete the contents (after you make sure they don't have any attachments open from Outlook they haven't saved elsewhere).
After that, the countdown to the next 100th file starts!