Odds are you haven't created exceptions in the firewall on the remote computer to allow SQL to communicate through it. Check out the complete instructions for enableing remote connections to SQL Express in the KB, http://support.microsoft.com/default.aspx?scid=kb;EN-US;914277.
FYI - This info is in the FAQ at the top of the forum is you need it again.
Ensure their is no network congesion between PCs you are connecting,
are they on same network segment !?
do you get ping acknoledgement when you ping your sql express from remote pc?
i would suggest you to refer http://support.microsoft.com/kb/905618/en-us also.
Hemantgiri S. Goswami