I am reading what you posted I thnk it may be a virus with your video slowdowns in browsers and games.
If it was just slowdowns in games it would be obvious because of your graphics card but its your browser too so I think your computer may be infected...
is a video on your hard drive slow to playback?
what settings do you use to play games?
the videos on the net use a tiny tiny amount of your video card
can you post your system specs with EVEREST
http://www.lavalys.com/
can you post your a picture of the performance tab on your taskmanger
and use CPU-Z for system temps
http://www.cpuid.com/cpuz.php
I'm thinking this may have to do with your cpu usage than your video card