Many networks support both TTLS and PEAP (being very similar), so you could try the latter.
SecureW2 EAP Suite is a TTLS+PAP client for Windows XP, though you'll need to find an older freeware version (it's a commercial product now).
For example, the Windows XP version of the eduroam configuration tool comes with some version of SecureW2, which you could extract using 7zip.