Validate server certificate

I know it's not a strictly cisco-based question and maybe is a dumb one, but I've got no other place where I can go :-(( In an environment with a wireless supplicant (windows xp native, aegis, odyssey, windows mobile or others), a cisco ap and a windows 2003 server with ias and certificate server I'm not able to make it work with the option "evaluate the server certificate" active, this applies however with both eap-tls and eap-peap. If I leave this option unchecked all works fine, if I check the option I see in the event viewer something like "wrong username or password". What's going on? Do I need a computer certificate in order to check the server one? Tnx, Tosh.