When opening an attachment directly from within Outlook you could get an error message saying that it can’t create the file and to that you need check the permissions on the folder you want to save it in. In most cases the permissions on the folder isn’t the issue but the fact that the folder is “full”. When you open an attachment directly from within Outlook it will first save a copy to a subfolder of the Temporary Internet Files folder. Cleaning out the folder will solve the issue.

Unfortunately this is easier said than done. The subfolder name Outlook creates (on installation of Outlook) in the Temporary Internet Files folder is quite random. It always starts with OLK but is followed by 2 random numbers or letters. Also by default you cannot simply browse to the folder to clean it out. Getting to the Temporary Outlook Folder can still be accomplished in 2 easy steps though.

Step 1: Locate the folder
The folder location is stored in the registry in the following key;

Outlook 97 HKEY_CURRENT_USER\Software\Microsoft\Office\8.0\Outlook\Security
Outlook 98 HKEY_CURRENT_USER\Software\Microsoft\Office\8.5\Outlook\Security
Outlook 2000 HKEY_CURRENT_USER\Software\Microsoft\Office\9.0\Outlook\Security
Outlook 2002/XP HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security
Outlook 2003 HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security
Outlook 2007 HKEY_CURRENT_USER\Software\Microsoft\Office\12.0\Outlook\Security

Step 2: Get to the folder
Open the OutlookSecureTempFolder registry key from the location provided in Step 1.
Copy the path from the key.
Open Explorer
Paste the address in the Address Bar and press Enter


Please see original topic here:
http://www.howto-outlook.com/faq/securetemp.htm