If it is DSL, your internet provider may require you to register your computer's MAC address with them. Though that's not very common. Who is your internet provider?
Have you tried setting up the router to use the username and password so that it doesn't need to be used on the computers themselves? (also known as enabling PPPOE on the router.)
I've seen that fix problems such as this, and I think it would be a good start.
Just log into your router, and look for a drop down menu that by default should show "dynamic IP address". It could also just be a list with the bullet select, but you get the idea.
Select PPPOE from the list, and put your DSL's username and password in there and apply. Hopefully after that all the computers on the router should be able to connect.