When you try to preview a file attachment in Outlook you receive the following message:  This file cannot be previewed. Try opening the file in the program in which it was created

You also receive an error when trying to open or save suggesting that there may be a permissions issue.

See below for explanation and solution:

When you open attachments from the email message before saving to the hard drive, Outlook places copies in the SecureTemp folder. This folder is a hidden folder under Temporary Internet files.
Background: When you open attachments on email directly from Outlook, a copy is written to a temporary folder on your hard drive so your the virus scanner can check it before it is opened. The folder Outlook writes this temp file to is under the Temporary Internet Files folder where IE writes pages to when you surf the internet. This offers more security as only your user account and the computer administrator account can view the files under this folder. The only way to avoid "losing" attachments in this well hidden folder is to save them to your hard drive, before opening. Few people do this (myself included).

These files are supposed to be deleted when you close the opened attachment, however this only happens if the email message the attachment arrived on remains open - this is either an opened message in a new window or in the reading pane. When the message is closed (or you wait to preview another message) after the attachment is closed, the temporary file is deleted from the SecureTemp folder. If you close the open message or display another Outlook item in the reading pane but leave the attachment open, the temp file is not deleted when you close the attachment. 

When this folder gets "full" you may have problems opening email attachments or embedded images won't display. If you open image attachments in Windows Photo or Image viewer and click next, you may see old images you thought you deleted. Delete the files in the folder to fix.
 
   
Find SecureTemp At start menu, Run, type regedit to open the registry editor.
Browse to this key:
HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\Security
OutlookSecureTempFolder value

(Change xx to reflect the version number of Outlook. 12.0 = Outlook 2007; 11.0 = Outlook 2003 and 1.0 = Outlook 2002.)

Double click on OutlookSecureTempFolder to open the value then copy the path. Paste it  into the address bar of Windows Explorer to open the folder.

This folder is normally hidden by the file system.
SecureTemp Locations In Windows XP the folder for Outlook 2000 to 2003 is at
C:\Documents and Settings\user\Local Settings\Temporary Internet Files\OLK*. The OLK directory will end in a series of numbers.

Outlook 2007 uses
C:\Documents and Settings\user\Local Settings\Temporary Internet Files\Content.Outlook\ 8A0VMD3A, where 8A0VMD3A can be any random characters.

Vista moves the Temp Internet folder to
C:\Users\username\AppData\Local\Microsoft\Windows\Temporary Internet Files\. Look there for the OLK or Content.Outlook folder. 
Move SecureTemp Folder 1.If it does not already exist, create the folder you want to use for your temporary files.
2.Browse to this registry key:
HKEY_CURRENT_USER\Software\Microsoft\Office\xx.0\Outlook\Security
OutlookSecureTempFolder value

Change xx to reflect the version number of Outlook. 12.0 = Outlook 2007; 11.0 = Outlook 2003 and 1.0 = Outlook 2002.
3.Replace the path with the path to the folder in Step 1.
Note: If the folder does not exist, the key will be reset to a new OLK* or Content.Outlook folder under Temporary Internet Files.