Wymann is right.
Time to upgrade, dude. My computer is virtually the same: (same motherboard, Athlon processor, 512 Mb RAM!) built it a decade ago myself and it was low end then. I can stream video all right most of the time, but I never use U-Tube so can't speak to that.
It's an ancient piece of crap, just like yours. It works for me because I use Ubuntu so I don't have to wait forever for start-up and I am mostly reading stuff on the internet rather than watching stuff. Even spending a few hundred bucks will get you a machine light years ahead of the one you have now.
The VLC suggestion is a good one, try that too.