|Thanks for the additional information. A few thoughts...|
I would strongly suggest completely disabling your Norton firewall while working on this issue. No need to add another layer of complexity and potential failure to the equation.
>>I opened ports 5800 and 5900 in the Norton Firewall
Yes, but you would need to open ports 5800, 5801, 5900, and 5901. Even if the VNC server on port 5800 is accessible from the Internet, the server won't be useable unless the session port is accessible as well.
>>Although, wouldn't forwarding the ports by-pass this?
Yes, it should.
>>I'm beginning to suspect that it might be something with my ISP.
I think this is reasonable at this point. What is the external IP address of your router? Some DSL providers use private-network addresses.
>>What stumps me is that www.gotomyvnc.com can see that a VNC server is active on port 5900.
This would invalidate the assumption that your "public" IP address is really private. However, since you are still apparently firewalling the VNC session ports (5801/5901) that could explain it.