2 nics to connect to router and switch

[itismycountry]

ok
this is my first time asking for advice online
usually for the past 10 years, i have been just reading
please be gentle, i will try my best

my only computer with one nic onboard and one pci
xp pro, core i7,3gbram

one nic connects to a router with internet
the other connects to a switch that's connected to 8 pc's (no internet)

situation: it works fine if i just plug them in my older computer
it was xp pro, 2nics,quad core 6600,2 gigs
now i have this new computer and when i plug both, the computer freezes
then i need to do hard reboot
if i plug one into ANY of the nic then boot- it works fine
while it is on, and i unplug one cable (no matter which one) and plug the other one- it works
so one works at a time

incidently, i use anyplace control to see the computers on the switch

can someone explain to me what to do or why was it working fine when i simply plugged both cables in my old computer with onboard and pci nics?

thanks