The larger a picture gets, especially with videos, the more stress you put on the graphics requirements. In addition, if the videos are online streaming videos, high-speed broadband is a must. The cheap, low-end internet connections just don't allow enough data to be transmitted fast enough..

You didn't really mention whether you have a good, dedicated graphics card, or whether you have a lot of RAM, but if you have neither of those, then you'll need to upgrade them if you want high quality video playback.

Either way, to diagnose much more, you'll need to give us more specific information.. Operating system? Graphics card type and amount of RAM included with it? Overall RAM installed on the machine? Internet connection speed? Etc.

Hope this helps.