You can have a strong signal, but your connection may not be a quick one. Or you may have other programs hoggin' up your prescious bandwidth (amount of internet you can access at a time). So you may have a STRONG SIGNAL, but you may have a slow connection SPEED.
To check how fast your computer is able to access internet, go to the wireless picture in the bottom right of your computer (the little screen with wavy little lines). Right click on it and click STATUS. The connection should be in Mbps. Many routers have a max speed of 54Mbps, but some others can go much faster. If the connection reads something like 18Mbps or 10Mbps or something significantly lower that 54Mbps, then that may be the problem.
But is you have a high Mbps, your computer may just need a good ol' cleanup of junk and temporary files and recycle bin gunk. CCleaner (ccleaner.com) is a good application to remove computer clutter in your internet programs. Downlad that and see how well everything's going!
Hope I was of some help!