Hi BAI, if you are using CRM4.0, the users can just type the name of the contact in the lookup field and it will automatically resolve to the contact in the database (assuming auto-resolution has not been turned off).
This is quicker and simpler than changing the default entity type in a multiple entity lookup search box. It is possible to change the default entity type, but it is an unsupported customisation. See here for an example:
http://umarkhan.wordpress.com/2007/10/22/how-to-change-the-default-entity-in-lookup-window/.
Regards, Neil