Peer to peer + Remote

Tom-Sr

I have a client who has one store with an XP Pro used as a peer to peer server and an XP Pro Workstation connected the internet(DSL) and connected together with a Linksys VPN Router. The workstation is in constant pos use. The XP Pro server is used intermittently during the day. The have subsequently added a remote store with an XP Pro laptop as the workstation. I have installed a VPN tunnel and the app won't run except prohibitely slow. Installed RDA on the remote and it works fine ... until someone tries to access the server.. for e-mail or other programs. At which point the remote is no longer available. How to do this with the capablilty of using RDA on the remote and both the other local computers without any kind of sharing disconnect. Will listen to any reasonable solutions, but I don't want to have to add a Windows server at the local store and use terminal services. I know I could add an inexpensive workstation at the local store and access it remotely but is this all that there is available for this problem. Thanks for your help.