Windows Client use different protocol set for VPN services, like PPTP or L2TP (that are quite legacy).
Cisco EzVPN client uses IKEv1 for initial handshake (policy exchange, dh for session key, and peer authentication) then actual Phase 2 takes place and encryption is taking place.
think of it, like making your laptop exchange word files with oven )
Now market of Remote Access VPNs is boomed with SSL VPN, that are supperior to EzVPN.
Yevgeniy is correct IKEv1 is the traditional standard RA VPN on the pix/asa, SSL is coming to the forefront, however because of the licenses SSL it is pricey, especialy if your using a professional CA.
Anyway we digress, it is possible to get l2tp working, here is the ASA how-to;