Remote Desktop (built into WIndows) will get you onto any of the other machines. It shows the other computers desktop in a window on your screen. You then have complete control of the remote PC as if you were sitting at it. If the remote connection is configured properly, you can even share drives, the clip board, printers, etc. between the two. With proper file sharing setup, you could easily transfer documents back and forth. Depending on your network speed, it can be fairly good response time. I usually have 2-5 remote desktop connections going at any one time at work so that I can manage my servers. It isn't difficult to setup at all.
Find the My Computer icon. Right click and select "Properties". Pick the "Remote" tab. In the middle of that screen click the box that says, "Allow users to connect remotely to this computer". That enables the computer to accept the Remote Desktop connection from other machines. Every computer should be set to not bypass the Welcome Screen. You want to have password protected accounts.
To login to the remote machine, go Start -> Accessories -> Remote Desktop Connection
From there you get a prompt for the target computer. There are a couple of ways to do it: by computer name, or by IP address. Either way, you will need to supply the user account and password of the remote system.