Internet connection doesn't work after a few minutes

Hi.
whenever I connect to the internet it works properly for a few minutes but after that it doesn't load anything, like I'm not connected to the internet however I AM connected and the icon in the system tray indicates that. When I click on my connections icon in the system tray it appears for a moment and disappears!
I reinstalled the windows and the problem was fixed but again after a day or two the same problem happened. Because of this I think it must be virus or something but the system I'm talking about is not mine, and I have just an hour or two to work with it! So I wanna find out how to fix the problem before I get there! what sort of virus is it? which anti-virus can clean it for sure?!
This is very crucial to me! please help me. Thanks.

should first talk to your internet service provider to find out if there are any problems with the line or any other problems at their end that may be causing the fault.