Quote from slipstick.com
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:
1.. 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`fs)
2.. Under that key, add a new string value named Level1Remove.
3.. 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.
If you are using this registry entry, a glance at Help | About Microsoft
Outlook will show Security Mode: User Controlled above the license
information.
If you prefer not to edit the registry directly, you can use one of the
various tools available