Error 721

I have a VPN server running on Windows 2003 server.

I have 8 client computers (Windows XP & Vista). All clients and server are
in LAN.

7 clients can connect well with no problem. 1 client (WinXP SP3) hangs at
'Verifying username and password', and then give error 721.

I've tried using different vpn dialer, but to no avail.

I read somewhere from Microsoft website that I need to reset the permission
to NetworkAuthority account by running a line of command provided in their
web, and I have done that as well.

To date, that 1 client still unable to connect.

All VPN connections are using PPTP. I have created enough ports in the
server.

Any suggestion how to fix this problem?

Thanks.