Freeradius, wireless access point and password authentication

Hello, I'm new to freeradius... I want to realize a sort of hot spot using freeradius, mysql (with a graphical interface creating users on it) and some access points (linksys, d-link, etc...). I correctly installed and integrated freeradius and mysql. Now the problem is to use the access points as NASes, and to realize an authentication system which prompts the client computers ( with Windows XP ) for a password in order to associate with the AP. I cannot use certificates to join the AP, because the client computers belongs to occasional visitors, and installing certificates on each client would be little practical. I made different trials, with no success. Thank you in advance for any help, and sorry for my bad english.... :-)