To find out where the folder is on your system go to start > run > type regedit. Be very careful here this is the operating systems central nervous system. DO NOT MAKE CHANGES TO THIS! in the regedit program browse down to this path HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security. When you get to security look for the value OutlookSecureTempFolder this will contain the path to the folder you will need to clean out. Find that folder and delete everything in it. Then restart outlook. If you had a lot of files in this folder you should notice outlook running faster.  I have only tested this with outlook 2003 and windows xp professional.