did you set each computer up such as
isp --> router --computers 1/2
check the ip address to make sure you did not set up the computers up with the same IP addresses, use winipcfg on the run command to check the ipaddresses, also make sure to set up the DNS correctly for each system.
did you make sure the IP and subnet mask are correct
such as
computer one 10.1.2.2/255.255.255.0
computer two 10.1.2.3/255.255.255.0
and the gateway.
are you using NAT on the router?
unless your using DHCP on the router for each computer.
your not trying to use the same MSN account at the same time on each computer are you?