connecting two domains via vpn

I am having 2 domains in different forest in 2 different offices. I am connecting these two domains via Vpn. for connecting the two domains i have created a trust between two domains.
But now the problem is when i am trying to connect a windows Xp machine to any of my domain when VPN is dialed, it donot allow me to connect it. I can connect windows XP machines to any of the domains when the VPN Connection is Off.
Secondly after connecting XP Machine to domain,I tried to Ping my local Domain, when the vpn is dialed, My ping to the domain is failed i.e request time out.
Pls let me known how can i logon to my remote domain via VPN with Windows Xp.