Using a 64-bit browser? That has caused a similar issue for a friend of mine that took us days to figure out. It's because there is no 64-bit version for flash yet.
Which takes me to my next idea, update flash player.
i updated the flash player dint work and btw i can watch videos on all other sites just not you-tube i just downloaded "AVG" can it be the problem it came whit some problem that i fixed and the firewall always blocks the internet so i have to disable it