1. Click Start > Run and type regedit.
2. Click Edit > Find and type OutlookSecureTempFolder.
3. When regedit has found the entry, it will tell you where your Outlook temporary files are stored. It will look similar to:
C:\Documents and Settings\username\Local Settings\Temporary Internet Files\OLKrandomnumber
4. Open My Computer and navigate to this location on your hard drive.
5. Delete all of the files in this folder.
6. Test open attachment
7. Restart Microsoft Outlook if not able yet
