Plug cable modem into WAN port on router.
Plug computers into router LAN ports 1 - 4.
Ensure computers are set up to automatically get ip address DHCP)
Reboot cable modem (pull power plug, wait 2 full minutes, plug back in)
Reboot router (pull plug, wait a bit, plug back in)
Turn on computers
That ought to work. If it doesn't, try a hard-reset of the router, which usually involves pushing and holding the reset button on the router for more than 5 seconds. Then go into the router and change the admin passwords.
If that still doesn't work, download and install the latest router firmware from linksys (it's in the router admin setup)
If all else fails, read the manual which may have you do odd things like cloning your primary PC's MAC address to your router.