Problem of Clients in Windows XP connect to DS Server in Fedora core 6

Hi,

We installed the DS server in Fedora core 6 OS, and several clients in both Fedora core 6 and Windows XP OS. The clients in Fedora core 6 are works well. But the clients in Windows XP professional version can't connect to the server. The Firework in the server machine is set off. Who can tell me what's the problem is?
Many thanks.
Another question is that does the installation of DS client also need a license?

Best Regards!

Bing