*Update your VideoCard drivers
*Run Norton Systemworks
*Close all non d2 related items in your system tray, excluding all mandatory programs ie.) Volume Control, Graphics Software.
Spyware, such as Stryder stated above will contribute to your lag problems. Also, adware like ACEgain, most commonly shipped with Winamp and GNUtella p2p should also be removed.
***EDIT***If you can login to battle.net I fail to see the use in acquiring new ports to use, seeing as though he can already connect, this solution would be best implemented if the user was behind a firewall and it was inhibiting his connectivity ;P