Doesn't sound like a fax problem to me.
Sounds like an Outlook Secure Temp Folder problem.

With Outlook closed, click Start>Run and enter %TEMP% in the Open field.
Click OK. You should see a Windows Explorer window with the folder list in
the left-hand pane. (If you don't see the folder list in Windows XP, click
the Folders button in the toolbar.) Just below the folder named Temp you
should see one called Temporary Internet Files. Click the plus sign to its
left. You should then see a subfolder whose name starts with OLK. Select
this folder so that it's contents display in the right-hand pane. Click
Edit>Select All and then click the Delete button (or press Delete or click
File>Delete). Close the Explorer window and restart Outlook. Your
attachment should open now.
--
Russ Valentine
[MVP-Outlook]