IE 9 ,10, 11 and youtube

my office just clean install 2 Win 7 pro + IE 9 and upgrade to 10 or 11.

and installed the new ver 18.029 fash player for IE and firefox.

But this 2 Win 7 pro, if play youtube , it will black screen and how me 'error occurs, please try later'

But I can play MyTV in those IE 9 /10/11.

But if I play Mytv and youtube in those Win 7 pro in Firefox, everything work fine.

anyone have Idea ? why only Clean install win 7 have problem.

My office all PC is win 7 and using update flash player, but it is ok.

they are Dell I3 , 4 G ram  + win 7 pro and IE 10 or 11. Only those 2 PC which I just recover + installed all update patch....but only cannot run youtube movie in IE.

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Now, I just go to youtube to select using HTML5 player for my IE for default.

seem ok now, mean my IE will use HTML5 to play youtube, but other still use Flash (mytv)