Connect two PCs to internet

Hello,

I would like to be able to access the net with two Pcs
at the same time.
One PC is a Gateway running Vista, second is a HP
laptop running XP. Have Netopia 2240N-Vgx modem and
a Linksys EZX55W VER.3. And Ethernet cables.
Connected Netopia, and both Pcs to to Linksys switch.
Got IP conflict, to access the net I must unplug one of the
Pcs for the other to work. Pleas help.

Thanks!