l2tp vpn pre-shared key on windows 2000 server

hi all, how can i configure a windows 2000 server to listen for l2tp ipsec
connection with a pre-shared key???
i have created a vpn server with rras and an ipsec policy with a pre-shared
key then i modify the registry
HKEY_LOCAL_MACHINE\System\CurrentControlSet\Servic es\Rasman\Parameters
adding ProhibitIpSec dword key with value=1 to disable ca authentication and
check for a local ipsec policy.
when i try to connect from a windows xp vpn client with shared key i'm unable.
can anyone tell me where i'm wrong???
thanks