Well I have been doing some thinking about your problem, and have a few suggestions for you as I went through this myself recently after a power outage...
First of all you can only have one machine on primary usually at a time so make sure the lap top is off or connected secondary to WinMX.
Also if your using Windows Xp the firewall has a habit of causing problems and its best to make an exception for WinMX in it.. go here for instructions on how.
Firewalls and WinMXBe sure your ports set in WinMX are the same as on the firewalls and router and that you have 4 separate numbers set in the router for the two computers...
If you still have this problem then try connecting using a secondary connection on the pc. If that works then is a conflict of ports somewhere.
If you dont need a primary and it works on secondary then be happy with that ... lol
Some network cards can only handle a certain number of primaries so not sure since you say it was a graphics card you added...
Let us know if any of this helped you out...