If you are using a Windows SBS 2003 or 2008 with a domain, and client computers are taking a while to login, make sure that you've got the DNS server set to the SBS box as the default.

We use a linux box as the dhcp server and gateway, and this small change to the dhcpd.conf reduced login times from 2 or more minutes down to seconds.

Wish i'd discovered that earlier!