I found this on the Microsoft website: there is a minor alteration that I had to do in order to correct the issue though.
Method 2: Delete the temporary files in the Outlook temporary folder
To delete the temporary copy of the e-mail attachment, you must first determine the location of the Outlook temporary folder. Then, you must delete the files in the Outlook temporary folder. To do this, follow these steps: 1. Click Start, click Run, type regedit in the Open box, and then click OK.
2. Locate and select the following subkey:
HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security
3. In the details pane, locate OutlookSecureTempFolder in the Name column. Note the path in the Data column. In this scenario, the Outlook temporary folder path may resemble the following path:
C:\Documents and Settings\user_name\Local Settings\Temporary Internet Files\OLKC\
4. On the File menu, click Exit to exit Registry Editor.
5. Start Windows Explorer. To do this, right-click Start, and then click Explore.
6. Locate and then select the folder that you noted in step 3.
7. On the Edit menu, click Select All.
Note All the files in the Outlook temporary folder are selected. In this scenario, the Outlook temporary folder is the OLKC folder.
8. On the File menu, click Delete.
Note If you are prompted to delete the files, click Yes.
9. On the File menu, click Close to exit Windows Explorer.
The alteration that I had to perform was this: While in the registry (HKEY_CURRENT_USER\Software\Microsoft\Office\11.0\Outlook\Security ) I selected the line that had the folder location, I then edited the folder name, saved the change, closed out of and re-opened Outlook, then tried again. Since all information was cleared out of the Temporary folder the file was able to be opened.
From what I have read, the reason behind this is: when you open a file out of Outlook it creates the file in a temporary folder named OLK# (# is variable), if you have to open a file with the same name again, Outlook adds a number to the file name up to 99. 99 is the cut off, when you try to open up #100 with the same name, this is the error you will recieve.
Hope this helps! (it took me all morning to get this fixed for my user!)