URGENT !!! Connecting two sites using ADSL

I have two offices (1 Km)at which we have ADSL connections of 1Mbps. At the main office we have a Domain server 2003 and 11 clients and at the branch office we have 10 Xp machines. I would like to connect all my branch office Pcs to be connected (and authenticated by)to the domain in the main office. What are my best options for connectivity. I would also like to know what will the requirements if I have to use VPN.