It is Really Easy To Do That
At First If You Have a Windows Server 2003 OS You Can Use Routing And Remote Accses To Creat VPn Server . and in Client You Must Creat A VPn Connection To Make a VPN Tunnel With Server and at This
Subject You must have a Valid ip of Your VPn Server ..And One another way you can is Using RRas Server To Dialing To Server And In This Subject You MusT Configre a RRas server In Your Main Office and asign a Tellphon NumBer For It and after That You must make A Connection In Your Branch office To Connect To Your Main Office But at last I Do Not Offer the Second Way becuse It Is Very Expenseive And You Will not Have Good Bandwith