Video cripples computer

When I use an application to watch video [WMP, iTunes, RealPlayer, DivX
Player], such an action seems to slow my computer to a near-halt.

Scenario: I double-click a [mpg, divx, avi, etc.] file and it opens the
corresponding application. The application attempts to play the video, but
cannot. Also, while failing to play the video, my computer gets agonizingly
slow [I can watch it "draw" dialogue boxes and right-click menus] to a
near-impossible-to-operate speed. When I am finally able to close the
by-now-unresponsive video application, the computer goes back to normal speed.

This also happens when I view videos online, using IE or another. However,
said-slowness only occurs when IE calls on another application to run the
video. Youtube, Myspace, etc can run their videos just fine. It's only when a
video requires WMP, Real, or QT that my problem happens.

Any suggestions?