I had this trouble before but I forgot what I did to fix it :c. We're using an SMC 8014W-G cable modem with a built in router and by god does it hate port forwarding.
Here's the port entry on the modem/router page:

And here are the details from ipconfig:
C:\Users\owner>ipconfig
Windows IP Configuration
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
Link-local IPv6 Address . . . . . : fe80::5dd1:5a5e:a67f:87e6%11
IPv4 Address. . . . . . . . . . . : 192.168.0.10
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Default Gateway . . . . . . . . . : 192.168.0.1
Tunnel adapter isatap.{1ADB147F-1663-4A0F-A6B3-0CB33A2C038A}:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Tunnel adapter Local Area Connection* 9:
Connection-specific DNS Suffix . :
IPv6 Address. . . . . . . . . . . : 2001:0:4137:9e74:3cb3:2814:e72c:4e44
Link-local IPv6 Address . . . . . : fe80::3cb3:2814:e72c:4e44%13
Default Gateway . . . . . . . . . : ::
Halp.