Just some thoughts:
Are the two Static IPs internal addresses, or are they assigned by your ISP?
If they're internal (ie: 192.168.100.x), and the cable modem is getting the single IP address to the internet, the WON servers for Half Life/Counterstrike will only see the game coming from a single IP address.
In that case, on the second computer, you need to add a Port command to the shortcut.
Example: C:\Program Files\SIERRA\Half-Life\hl.exe" -port 27027
You'll need to use a different port for each additional computer, if you add more in the future.
I had the same problem where one system would log onto an internet game, and the second wouldn't connect. The Port specification fixed it.