Recent Outlook Versions Outlook 2003, Outlook 2002 and Outlook 2000 SP3 (but not Outlook 98 or earlier Outlook 2000 versions) allow the user to use a registry key to open up access to blocked attachments. (Always make a backup before editing the registry.) To use this key: 
Run Regedit, and go to this key: 

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)

Under that key, add a new string value named Level1Remove.

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