Very slow login of XP Pro on NT4 network

I have 4 XP Pro laptops logging in to a NT 4.0 domain with 2 domain controllers. The laptops can browse the network fine, use resources, see mapped drives, etc. I can ping all servers and resolve the names via our ISP’s DNS servers. From the time a password is entered in the logon box and you press Enter, it takes over 2 minutes to complete the login and bring up the desktop. The servers are 1 Ghz and 600 Mhz, the laptops are 1.6 Ghz. Network is 100 Mbps switched environment. This is driving mecrazy!