Press Ctrl-Alt-Delete. Click on the 'Processes tab. Look down the 'CPU' column. The only big number (probably over 90) you should see ought to be against 'System Idle Process'. If any other program has a high number it's that program which is hogging all of your processing power. (Report back, on this thread or on a new one, to tell us what it is).
However Youtube buffering is far more likely to be due to a problem with your connection to the internet. Start by running a ping test here:
http://pingtest.net/
Click on 'Begin Test' and wait for a minute or two for your test result. You're seeking a Grade 'A' or 'B' result. Anything worse suggests that there's a problem with your phone line.
Then run a speed test here:
http://www.speedtest.net/
(Ignore the advertising hype, where it says 'Before you begin. . . ', simply click on 'Begin Test'). You're seeking a download speed of at least 2Mbps. (That's probably the lowest figure for reliable Youtube downloads). If you get anything lower, there's a problem with your broadband connection. It might be at your end (e.g. using an extension lead from your phone socket or having a wireless device too far away from your router) or it could be a problem with your ISP.
Also, download, install and run the free version of Malwarebyte's Anti-Malware. It might find a rogue program that's causing problems:
http://www.malwarebyt...cts/malwarebytes_free
Chris