Windows XP, mschap and the NAS

I think I have set up freeradius correctly to use MSCHAP, but the only way I thought I could test it is with the real client, i.e. Windows XP 'dialing in' (via a IP wireless VPN through EVDO). Running FreeRadius in debug mode, and using the default Windows XP settings I only get a CHAP request come through. Is it possible that the NAS sitting in between the client and the internal FR server can only pass on MS-CHAP, or is there some negotiation that goes on as to what the client sends off? When I force it to use MS-CHAP in Windows XP, I don't even see the request come through to FR. Any help on this?