if i happens when you run too many programs, it should be RAM
problem.
What is actually running is not what you can see. You may have many
stuff running in background beside IE and media player.
like some system applications that you need to run it in background
by default.
And things in your system tray that you are not using, but it still
takes up some of your RAM.
and maybe some spyware that you dont aware that it is running.