If you want to view sites like Youtube you Must install the flash player. Also It's NOT a Microstoft product, it's originally from Macromedia now owned by Adobe.

Many sites use Java to detect and redirect or give a message if the flash player is not found on your system.

What broswer are you using, they support Firefox, Safari and Opera for Mac?
Adobe - Web Players

Go there re-install from one of the supported browsers. IF you still have an issue check your security settings in the browser.

FYI
98.6% of computers have flash player 7 or above installed.
Adobe - Flash Player : PC Penetration