First, be sure you're going about this the right way. Go into (hard drive name)/System/Library/CoreServices and put Screen Sharing in your dock. This is the built in VNC / ARD tool Apple uses. When you run it you'll be asked for a "Host". What it's asking for is the name of the computer to connect to or where it can find it. If you're running internal DNS, set up an A record and use that name. Otherwise, use the IP address of your PC. You can get this from any PC by clicking on Start and in the run box type in the letters "cmd" (no quotes) and hitting enter. You get a black box. Type in "ipconfig" and hitting enter. Look for the number after "IPv4 address". This is the IP address of your PC. Put this number into Apple's screen sharing utility. You should not be prompted for a name, just a password. If it's still asking for a name and pw, it thinks it's connecting to a mac via ARD and not VNC.
That takes us to plan B, which is to google on "Jolly's Fast VNC". There are other clients out there but I like that one best for ease of use. If your PC does not show up automatically in that, click the plus on the bottom to add a server and use the IP address you looked up above.