Did you move the DHCP server from your 2003 box to your 2008 box?
If you did, did you shutdown the DHCP Server on the 2003 box?
If you shutdown the DHCP server on your 2003 box and you're still receiving the "No Login Server is available" message on your desktops, you may also want to try:
- Click Start then Run
- Type "cmd" ( minus the quotes )
- In the command prompt window, type ipconfig /release and press enter
- Next type ipconfig /renew and press enter
This will force the desktops to renew their IP addresses from your 2008 DHCP server and allow your desktops to login.
I would also recommend removing the DNS server entry from the DHCP Scope options for your 2003 Server.
Hope this helps,