Youtube does use HTML5 player as well as Flash Player so the video's may be playing in that. You can right-click on the video and it'll tell you what player it's using.
Please provide the C:\Windows\System32\Macromed\Flash\FlashInstall.log file using the instructions here, The specified item was not found.. If this is a 64-bit OS, please also provide the C:\Windows\Syswow64\Macromed\Flash\FlashInstall.log file.
Please do the following:
- Close all browsers and apps that may use Flash Player (e.g. instant messaging clients, Pandora and the like).
- Run the uninstaller posted at the bottom of the Download Adobe Flash Player 15 Beta for Desktops - Adobe Labs page
- Reboot the machine
- After logging back in, check the C:\Windows\System32\Macromed\Flash and C:\Windows\SysWow64\Macroemd\Flash directory.
- FlashInstall.log file should exist in both directories. mms.cfg file should only exist in the 'syswow64' directory. Depending on what other Adobe software, if any, you have installed there may be other CFG files and a FlashPlayerTrust directory. If there are other Flash Player files listed, take a screen-shot and post them using the instructions previously provided.
- Run the installers posted to the bottom of this page, Installation problems | Flash Player | Windows
If there are any errors, take a screen-shot of the error dialog and upload the screen-shot and the FlashInstall.log file from both 'system32' and 'syswow64' directories using the instructions previously provided.