If you want to access a Windows computer remotely, you can use the built-in Remote Desktop functionality – if you’re lucky enough to be running a professional edition of Windows XP or Vista.
OS X also comes with remote desktop support in the form of a built in VNC server, which is calls “screen sharing”. You can enable screen sharing in the Sharing preference pane. Unfortunately, the built in VNC server and viewer in OS X are very limited, especially if you’re used to the Remote Desktop functionality of Windows.
If you have a few hundred dollars to spare, you can purchase Remote Desktop for OS X. Otherwise, you can enhance the built in viewer and run your own VNC server for free. If you are connecting to OS X from Windows, there are many VNC viewers to choose from, such as Tight VNC, Ultra VNC, and Real VNC. The servers for Ultra and Real VNC are commercial, but the viewers are free. My favorite is the Real VNC viewer from their Enterprise edition. (You can download the viewer separately.)
My favorite option lacking from the build-in functionality is “Adapt to Network Speed”, which automatically adjusts the image quality to the speed of your network.
P.S.: If you want to access Windows from your Mac, get Microsoft’s Remote Desktop for OS X. Another free option for both Mac and PC is LogMeIn.com