Hi Chris
Well I'm not in a cave in the himalaya's but reading through what you have told me in your post i would say one of two things which you might not like but i'm brutally honest.
Firstly dont rely on windows to be truthful. it quite frequently isn't.
On your desktop machine check your onboard LAN connection is enabled by using the BIOS. try connecting the ethernet cable to it then boot into windows and see if you can establish a connection between the two machines. you might need the benefit of Windows New Connection Wizard to help you. Enter control panel and select network and internet connections then select SEt up or change your home or small office network. this launches the network connection wizard. do as it tells you and complete the necessary fields.
In retrospect to your add in NIC - did software come with it? i'm talking about drivers here. If so did you install them properly. Its unlikely that both your PCI bus slots are dead. if theres no lights to the card the card maight well be dead and buried.
let me know how you get on with what i've suggested.
BTW: are you planning on accessing the web on both machines? if so get a network Modem/Router and some extra cat 5 cables.