SQL2000 use a particular TCP port, SQL2005 asked you to select one at install time.

We put the following reg key on our clients, don't know if this is the best solution, but it is what we're doing.
[HKLM\SOFTWARE\Microsoft\MSSQLServer\Client\CONNECTTO]
"%DATABASE_SERVER%"="DBMSSOCN,%DBSERVER_IP%\\%DB_NAME%,%PORT_NUMBER%"

Bits between the %% are what you'll need to change