So for the past week my PC has been freezing whenever I try to watch youtube videos (plus any web player). Before it happens, the video plays for a while - but has no sound. I can still listen to music via itunes so sound isn't an issue there. So I usually press the power button until it turns off then reboot it and start windows normally. After say, 3 or 4 times of doing this, youtube finally has audio. That being said, audio doesn't kick in for 5 seconds in on the first video I try. After this, my PC runs fine and when the time comes to shut down, it freezes again. I have no idea what's causing this.
Open task manager and click the processes tab. Under options tab, select 'always on top' for now. Move it out of the way of the video. Check processes that are running before you start the video. Make sure you expand the task manager's window as long as possible and view all is selected. Run the video then check task manager for ram and cpu usage, and any other, before it freezes. Continually closing down the PC via the start button isn't good for the mobo. If all looks fine then I'd hazzard a guess that the PSU might be failing. If you have any USB periferals, other than monitor and keyboard and mouse, disconnect them after shutdown. Boot up again and see if the PC works better.