I'm constantly accessing different computers, but I don't need to get up from my chair to swap PCs. Instead of walking between them, a VNC (virtual network computer) setup lets me log into remote machines that are across the room or around the world. Now I can manage software installations, see if a download is complete, or just troubleshoot from anywhere.
Windows XP Professional, and Vista Business and Ultimate include this functionality, called Remote Desktop. (Lesser SKUs can connect to those operating systems, but they can't host a remote computer.) You can also set up PCs with the open, VNC protocol, using a range of software. But those routines often require port forwarding and other network management. I've shirked that level of network management and sampled LogMeIn, which uses its web server so you don't need to know IP addresses or other networking details. Plus a new, albeit paid, iPhone client [app link] drives it all from anywhere.
Just install free host software on the remote PC, and add an optional client installation on your controlling side. You'll then log into the website and see a list of your configured PCs (or Macs). The service works inside the web browser, and other than moderate delays for screen redrawing, it's just like controlling a PC normally. The paid service adds a few extra features, such as simplification of file transfer to a remote computer, but you might never need that upgrade.