DNS Server Error: "Windows can't communicate with the device or resource"

Please help me out to get rid with this problem as quickly as possible, I have installed a network installed at my place and from last few weeks I am constantly facing the problem of connection failed. The problem is so frustrating that whenever I am set to do an important task at that time only my internet connection gets blocked and I have to wait for few minutes until the connection gets started again. As, suggested by my friend , I decided to run the diagnostic test to check what was the problem but I got an error message saying "Windows can't communicate with the device or resource(primary DNS server)". So guys can some one help me to know what is really going on and why I am facing this kind of problem and more importantly how can I be able to solve the problem.

Check the router whether it is working perfectly or not, if possible try out some different router of any one and see if the problem still persists or not. If it solves the error than its good for you but if it doesn’t then I think that you will have to change your router and go for a new one. Also, as far as the slow internet connection is concerned I think that you should contact your service provider and see if they are able to help you . It can be possible that the problem be from their end.

Why need a router if you are having just one computer at your home, you can also directly connect the modem to your computer and can have a direct access to the internet from there. I also think that the problem is something to do with the router and having that replaced might well solve the problem. Also, you should check for the ports and the connection that is there should be no loose connection because that can also be the possibility of the slow internet connection.

Thank you friend for your quick reply, I have done exactly as you all have said, I contacted by Internet service provider and have asked them about the problem but they told that every thing is correct form their side also, I told them to give me a new router and with the new one also I am facing the same problem again. Also I would want to inform you that I have been installed the network at my work place that has 15 computers so I have to use a router to connect the internet connection to all of them, but unfortunately all of them are facing the same problem. Also, if I connect the same with the modem to only one computer than I will get the internet running smoothly. So can some one please suggest me to know how can I be able to solve the problem.