Supply more information please.
OS running on all affected systems
Make\models of all equipment (routers, network cards, modems).
Wired or wireless?
What wireless encryption is in use?
Browsers being used (Firefox, IE, Safari, Opera,...)
Open up a command prompt (Start > run > cmd)
Type the following bolded commands and post the results for each affected machine.
ipconfig /all
ping www.google.com
ping 74.125.53.99