1. Open REGEDIT.EXE and go to Edit -> Find... In the Find dialog box type "OutlookSecureTempFolder" without the quotes and locate that registry key.
2. That key will contain the actual folder location, and will look like:
C:\Documents and Settings\%USER_NAME%\Local Settings\Temporary Internet Files\OLK#\ (where # is a random letter or number)
3. Copy the location of that folder.
4. Click on Start -> Run... and paste the folder location from step #2 then click OK.
5. Windows Explorer will open that folder. Please, delete all files present.
6. Restart Microsoft Outlook and check if you are able to open the attachments