PC to Mac - Remote Desktop
VNC
Recent versions of Mac OS X include a VNC server built-in, analogous to Microsoft's Remote Desktop Server (included in Windows). This can be accessed via the System Preferences &gt; Sharing &gt; Apple Remote Desktop option.
There are various free VNC clients available for Windows, most notably RealVNC.
While there are other options, VNC benefits from being a very mature graphical remote-access protocol with official support from Apple and good-quality client programs.