setting up vpn l2tp on w2k server

Hi all!
Currently I have w2k server and a winxp client working without
problems with PPTP vpn.

I want to migrate to a L2TP setup.
So far I've installed the certificates services on the server which
issued me a certificate AFAIK.
On the client I've accessed the IIS server of w2k and requested a user
certificated which then was installed without probs.

The trouble is when connecting to w2k server, through L2TP vpn, I get
this error:

Error 781: The encryption attempt failed because no valid certificate
was found.

I've seen some posts around with this error but haven't been able to
solve this out.
Hope you can shed some light.
Thanks!