Windows Firewall is the first place I'd look. Port 5900 + display#
PS:
Neat, seldom used command: "wf.msc". It gives you a LOT more control than the brain-damaged GUI in Control Panel.
PPS:
The easiest test is to turn Windows firewall OFF. If VNC suddenly connects, you've found the culprit. Just tune your firewall rules. Otherwise, you'll know to keep digging.