|So all are connected by wireless? Whats your signal strengh like? |
Have you considered reseting the router/modem to default settings and reconfiguring your router/modem?
Are all the computers up to date with the latest Service pack and updates?
Do you think it could be a problem with your dhcp leases? Check your dhcp config in your router, remove the leases on the router and on the laptops (start-run-cmd-ipconfig /release) and restart the dhcp service on the router then the laptops (start-run-cmd-ipconfig /renew).
Gives thes a try. Try blowing the router config away as a last resort after you have tried the others.