iis nt authentication

Im in the middle of setting up Liberum Help Desk, nt authentication works when I browse to the logon page from the server running iis but when I browse to the logon page from any xp client i get a "Unable to obtain username with NT authentication" message, how do I get the clients to logon?

I`ve just logged off the server running iis and back on again and get the "Unable to obtain message" on the server aswell.