I'm assuming all you want to do is local administration, that is, from within your LAN and not through the Internet. Which version of VNC did you install on the PC side? RealVNC v4.0 or TightVNC?
First thing you have to make sure is that the service (VNC) is running. There should be a little white icon on the taskbar meaning it's running. It turns black when someone is connected to your computer.
Second is if the PC is actually allowing you to connect to the service. By default, it should accept all incoming requests, but you can be sure by going into the configuration options and granting/revoking access to a particular computer or an entire network.
Third thing to check is if you have a firewall installed on the PC side. Are you using a program like ZoneAlarm or Norton Personal Firewall? Make sure that the firewall is accepting incoming connections to TCP port 5900.
If you've checked all that, you can first try a simple telnet test to see whether the Mac is able to reach port 5900 of the PC. You can go to the Terminal and type telnet <ip.address.of.pc> 5900
. You should see something to the likes of:
Connected to portal.in.svi.ph.
Escape character is '^]'.
If you don't get any output at all, either your PC isn't allowing incoming connections to that port or it isn't listening at all.