I know it's been a while, but this is an actual tech tip that I need to keep track of... Instead of googling for it every time it comes up, I'll toss it up here. ...and google for it in the search bar at the right... anyway...
In Outlook 2010 if you are getting a lot of the same attachments, say from a voicemail system, and all the attachments are named the same thing, eventually you will get over 100... Once you get 99 of them, everything after that will give you the error "Cannot Create File:..." when you try to open it. Here's the fix:
1. Click Start > Run and type regedit.
2. Click Edit > Find and type OutlookSecureTempFolder.
3. It should be something like: C:\Documents and Settings\username\Local Settings\Temporary Internet Files\OLK###
4. Browse to this location in explorer.
5. You can delete only the voicemail files if you'd like, or delete all of the files in this folder.
6. Restart Outlook.