I have comcast.net as my IPS , and 2 computers to run it to?

I ran the cable to the modem and from the modem I ran Ethernet to a LINKSYS 5port workgroup switch [model # EZXS55W] .Then ran ethernet from the switch to both computers independently on the primary I ran it to a [Lan Card added to a PCI slot] this unit works perfectly. The 2nd unit is connected to an Ethernet on the Motherboard this unit will not link up .Do I need to install a Lan card ,I thought all Ethernet plugs worked for all data transfer ? what am I doing wrong here ? I am on-line with the primary unit right now with the modem and switch in line . I need some help here friends ,I can fix most anything else but when it comes to a computer I am like lost ! without true guidance.