So either way you'll need 3 computers at each office. If you do not have the Professional Version of Windows at the current office. I strongly suggest you get it on the new ones you buy. You'll want the ones with professional at the main office (where the main computer is.)
What everyone suggests is to load LogMeIn on each of those 3 computers at the main office. Then remote into them from the second office. That's about it.
I think Remote Desktop would be better. Enable remote desktop at the main office and remote into it from the second office. I would recommend doing both LogMeIn and Remote Desktop to see which one you like more.
Personally, I've found Remote Desktop to run much smoother even when the internet is very slow.