Outlook 2003 allow the user to use a registry key to open up access to blocked attachments. (Always make a backup before editing the registry.) 

To do this, follow the next steps:

1. Open the registry editor Start -> Run, type regedit and click OK.
    
2. Expand HKEY_CURRENT_USER\Software\Microsoft\Office\10.0\Outlook\Security (change 10.0 to 9.0 for Outlook 2000 SP3 or to 11.0 for Outlook 2003) (see picture 1).
    

   Picture 1

   
   
3. Under that key, add a new string value named "Level1Remove".
    
4. For the value for Level1Remove, enter a semicolon-delimited list of file extensions. For example, entering this:
   
   .mdb;.url
   
   would unblock Microsoft Access files and Internet shortcuts. Note that the use of a leading dot was not previously required, however, new security patches may require it. If you are using  "mdb;url" format and extensions are blocked, add a dot to each extension. Note also that there is not a space between extensions.
    

5. The modifications you made will be in effect after you restart Outlook.