video problem

I'm stumped! My wife uses IE9 and her machine and has a problem that I can't solve having spent several hours trying.

Her friends send her links to their YouTube videos but they won't play --- hang up after starting for a few seconds. The same video will play fine on two other machines that are set up the same (I think). And same problem occurs with Firefox when I tried it so it appears to be a more general problem than something specific to IE9.

I've checked through the internet options, plus googleing for info, and have the right video players available for YouTube.

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