RSA Authentication Manager not authenticating!

We're testing RSA SecurID tokens in the office using Authentication Manager 6, which nobody here has any experience of. I've built a Win2K3 domain controller and set up Authentication Manager on it, which all appears to be working, and I've setup an XP client machine which, when you test authentication against the server works perfectly. However, when logging into the XP machine, it doesn't seem to use the account I've setup in Radius, and is instead still using the Windows Authentication, despite logging in through the RSA Gina. Any ideas from anyone, as I say I've been kind of dropped in at the deep end here!