Enable Remote Desktop on the systems that you want to access. Now you can log into them from anywhere on the LAN quite easily. Just be aware that Remote Desktop will forcefully logout any user on the remote system when you log in.
( I suppose MS is too afraid that people would start using one PC for multiple concurrent users if they allowed you to login in the background. There are hacks available online that allow you to do background logins, but I don't use that sort of stuff on customers PC's. I do have it installed on a couple of systems at home and it works great, though some apps complain that they are "already running" when you try to open them as the background user if they are indeed in use by the primary user).
You can also use UltraVNC, which grabs the remote screen without logging the user out. I use this a lot as it allows me to actually see what a user is doing when they have a problem. I've found it to be far more useful than trying to have a non-technical user explain exactly what they're doing. (I often hear "I got some kind of error when I tried to log in". Me: "Uhh, what did the message say?" What were you trying to log into? Windows? eMail? A database app?)