Lost Internet Connection trying to fix network

I've been trying to get a network connection running between 2 PC's at home.
Everything worked fine on the 2nd PC (could access files/printer on the main
machine) but I couldn't get the network running on the main PC. Both share a
cable modem internet connection via a Linksys router.

Yesterday I started playing around with various network settings on both
machines...with the result being that I've somehow managed to lose the
internet connection to the 2nd PC (and still can't get the network
connection running on the main machine!!!).

I've tried a System Restore on both machines (both running XP) but still
can't get online on the 2nd machine although the internet connection on the
main machine is fine. I've checked the network cables and switched the
router off and back on with no success.

Can anyone suggest any settings I might have changed that could have
affected internet access to the 2nd PC??

Thanks