Does XP natively support LEAP without Cisco aironet cards, or are the Cisco adapters required?
Well, you are indeed going to need a certificate on your AAA server(s). I know that Cisco ACS servers can generate a self-signed certificate if you don't want to buy one from Verisign nor set up your own CA; I presume MS IAS can do the same thing but have not tried it.
Other than the certificates, it's not all that complicated: Make sure your AAA server will handle PEAP; make sure your APs or WLCs are set up for 802.1X; make sure your clients are configured properly for WPA(/2) Enterprise and PEAP. Disable automatic use of Windows login if necessary.