turning off the nic in the bios is very very simple as its usually just one option usually called onboard LAN or onboard ethernet. if your managing to follow the instructions of the other posts then trust me its simple
))
my 2 year old could do it if i showed him what to do
its even simpler than adding a gfx card and ram to a pc which my GF has done herself.
as per Dai's post even if you added another hardware NIC then you would still need to turn off the onboard one just to be sure that its not a bad nic causing both the network and mouse problems