Things you could do when you get disconnected is try to ping your router (192.168.1.1), ping something outside (like www.yahoo.com) ping your DNS 95.77.94.88. If you can ping your router, then your problem isn't your network card. I bet you won't be able to ping yahoo though, but we"ll see.
Could you do a tracert yahoo.com (or any other address) to get your gateway, and then once you have it, it would be great to see if you can ping it when you have your problem.
Oh wait: when you say your brother has no problem, you mean his computer is connected the same way? Cable to your router, and he never gets disconnected?
Have you installed or updated something, like an antivirus program?
Have you checked the system events? Open a DOS command window and type eventvwr.msc and check also the application events.