Why does my internet not work?

My computer that I bought almost 5 months ago has had internet ever since i bought it until randomly, it wants to not work anymore. I am running windows 7 so when I try to diagnose it, it tells me whats wrong with it. It is telling me "Can't communicate with Primary DNS Server". When I try a computer in another room, which is all hooked in the same router, the internet works fine. So I then try to use an older computer in my room. That also worked. So seeing these two work makes me believe that its that actual computer. I have tried things that I have found but they are not working. One thing that was suggested was to reset Internet Protocol (TCP/IP). When I follow the steps, the cmd is telling me that it fails because it needs to be run by the admin. There's only one user on that computer. Any suggestions would be great. Thanks.