Internet stays only for few minutes on starting desktop

I have windows 7 desktop and couple of other devices including tablet and laptop.

My windows 7 desktop is hard wired from router and the rest all devices are wifi connected.

All other devices including cell phones are able to access internet using wifi. But my desktop is giving problem.
When I start or restart the device, internet comes but in a few minutes, internet goes away and I need to restart the machine again.
I tried few browsers, IE, Firefox, Chrome, etc... but there is no help in switching the browsers.

Please help ! What could be the issue here ?