Internet Sometimes Does Not Work.

Ok, where to start. I am on my brother's computer right now, which is the computer that has the problems. I also run my laptop on the same internet connection, and there is never a problem with my laptop. Sometimes when I start up the computer, it shows the internet is working, but I will open internet explorer, aim, etc. And they will not work. I will restart my computer, sometimes that works, sometimes it doesn't. There has been a time where I had to restart the computer about 5-6 times before it worked. I don't know what it wrong with it. Also, sometimes if it does work, as I continue using it, I will open another tab (window, etc.) and it will not work, I think it gives a connection error. BUT the window I am currently working on will continue to work, I just cannot open a new window becuase the new window wont work. And, as I said, my laptop never has this problem, my laptop always has connection. It's weird, so I really don't know what is wrong. Please if anyone could help it would be much appretiated.

Thank you.