youtube videos wont play & show up as a black rectangle

i use firefox, vista. i watch youtube every day, but since last night no videos will play, it just shows a black box where the player should be. i've unistalled & reinstalled adobe flash player, i'v installed the pluggins in my browser, i've cleaned the cache, ran CCleaner & disk clean up, reboot.... still shows a black box where the player should be. how can i fix this? thanks == URL of affected sites == http://youtube.com