One pc can connect to internet while the other one cannot!

Hello,

Recently I installed a WiFi router (linksys) , i don't know why the sometime the router connect to internet it won't work and sometime its work. then i checked through CMD-> ipconfig... I found that if I can connect to internet the gateway IP is (192.168.1.1) but if I can't connect to internet, the gateway ip is (192.168.0.1).
which is
192.168.1.1 (modem IP address)
192.168.0.1 (router IP address)
sometime i can connect to modem IP, sometime will not. I wonder why, any setting to prevent the pc to connect to only modem IP address... Please help!!
thanks