First of all - welcome to the site

Everyone here is very helpfull - I am sure that we can sort out your problem...
O.K. One ot two questions for you.
1. Are the clients (pc's) using dhcp or have you set them up manualy ? If you are not using dhcp you may have missed the default gateway entry or the dns entry's
2. Are you using a set top box to access the net ? If you are you need to insure 2 things - 1. you are using a cross over cable between the set top box and the router. 2. You have set the router to clone the mac address of the network card you had it plugged into or register the new macx address online.
I hope this is of help - If there is any more info you can give us it would be helpfull. You could run ipconfig /all from a dos prompt on all the machines and post the results - This would help.
Cheers