I believe that it is common for the CPU to reach 90°C under heavy load. However a simple video should not bump an i5 into that region. Depending on the footage and format you are playing back you might want to try a different player (vlc mplayerX). If you are playing back flash HD videos in your browser (especially multiple streams simultaneously), that behavior might be common (due to the bad flash performance on os x).
So much for the software.
Secondly you should take care of the outer temperature (e.g.is your computer in a very hot room) and even more importantly where your notebook is placed. e.g. does it sit on a carpet or any other good insulating soft cushiony surface that prevents heat from leaving your computer.
Thirdly you might want to take a look in the activity monitor and see whats using your cpu. (Maybe some background application e.g. spotlight indexing new pdfs)