pptp is shutting down my local network access to the machine

I have two machines on my local netork, i attach to a pptp network as a workstation on machine A, now i can still access machine B from machine A, but if i go on machine B i can not access machine A, can not even ping it. I hope there is a way arround this. As soon as i disconnect from the pptp network on machine A, then machine B can access machine A.