Internet down….Wierd situation

Our internet connection here always down. I try to check the connection. Its ok. I can ping www.yahoo.com through command promt. But my browser and proxy server cannot detect the internet. There is no IDLE time setting inside the modem. I have proxy software . I try to restart the software but it still cannot work. I think it not because of the software but it because of the OS it self. Some of my friend said I need to format back the server but I dont think it the right way to solve it. When I review the event viewer it keep saying I got a error userenv and event ID 1000. Do you have any idea how to solve it. Please my job are depend on you all….