MAC based auth

Hi, read manuals but i don't know how can i use mac based authentication. I used eap-tls and username/pass. It worked good. but when I not log on to the Windows server, I want to authenticate the computer, cos my server services have to reachable. pc try authenticate using name like host/PCNAME but i don't know what is a password... I think if i use mac address based auth., i don't need username/pass, simply enough a mac address. or is it a wrong idea? how can i set it to use just mac addresses to authentication?I want authenticate the hardware not the user(cos the user is not logged on). configuration: newest freeRadius, cisco switch, win xp thanks GH