It happens on Youtube and in Windows Media Player. Quicktime, oddly enough, isn't playing any videos at all; I never actually used it for anything before. (This may be a codec issue. I'll fiddle and see if I can get it to work.)
Update: Apparently, Quicktime for Windows is incompatible with Divx/Xvid codecs, which is why I can't play my .avi files in the Quicktime Player. There is a codec called "3ivx" that is supposed to work, but the creators charge for it.
For YouTube, try right clicking, choose 'Settings...' and uncheck 'Enable hardware acceleration'. Any change?
This is a thread where people can ask questions that they would ordinarily feel embarrassed for not knowing the answer to. The previous thread is at close to 500 comments.