Mac OSX comes with a VNC server service built-in. It however is not enabled by default for security reasons. To connect to this server to control the Mac computer, you need the client or viewer installed on the machine you are physically at. If this computer is also a Mac, the viewer is already installed. If this is a Windows PC, you need to get a viewer such as TightVNC.
http://www.tightvnc.com/download.php
This download has two parts: the server and the client (viewer). If your goal is not to control the machine the program is installed on, then make sure you only choose the "viewer only" option. From a security standpoint, there is no reason to have the server running when it is not needed.
Once the viewer is installed on the PC, just simply launch the program and enter the IP address of the destination Mac. If it does not work, it is because you need to turn on the “screen sharing” feature built into Mac OS X.
Click the Apple logo at the top left corner of the Mac’s screen and select System Preferences.
Click the Sharing icon to open the Sharing
Click the checkbox of Screen Sharing to enable screen sharing. (Remember to turn this off when the remote session is done).
Click the “Computer Settings....” button. Pay attention to what it says immediately on top of this button. It lists the current IP address of the Mac computer. This is the IP address that you would put in the TightVNC viewer on the PC.
Click the box VNC Viewers May Control Screen with a Password. Type a password into the box and click “OK.” This password is mandatory, used for the VNC session only so it does not need to be the same password you use for logging into the Mac computer.
Try again on the TightVNC viewer on the PC. It should work now.
If this is done over the Internet or the Mac is behind a firewall, you’ll need to set up port forwarding on the Mac's network router. Forward incoming traffic to port 5900. In situation like this, you should use the router’s external WAN IP address to connect instead of the IP address displayed in the Screen Sharing window seen previously on the Mac.
Incidentally, the VNC viewer on the Mac is the network share access program. On the Desktop (Finder), click on Go at the top of the screen. If you do not see this Go, just click on the Finder icon on the Dock. Choose Connect to Server. Type in vnc://IP address of the destination Mac in the text box. Connect.