What version of MT ROS you have?
I have succeeded with version 3 of MT. Please update to the latest stable version and try.
You have to define L2TP server and secret (your username and password) in your MT in PPP and also you have to define IPSec peer with adress 0.0.0.0/0 (ATTENTION!: not 0.0.0.0, but 0.0.0.0/0) if you have dynamic ip or you do not know what IP adress of vpn client would be, then enter your preshared key, and also the easiest way is to setup generate policy to yes.
With such basic setup and new MT version you should be successful. Also, you have to setup same username and password and preshared key in you Win XP client machine.
Only big problem that is left to me to solve is that this setup does not work when client machine is behind NAT, and the reason for that is IPsec, so if somebody knows what to do, please suggest! That is my problem: http://forum.mikrotik.com/t/mtik-l2tp-ipsec-vpn-server-for-win-clients-behind-nat/25451/1