Why is my older upgraded computer is slow when playing videos and games?

Its an older computer but it has a upgraded video card ATI, Soundblaster sound card, wireless card upgrade U.S. Robotics, ram upgrade 1.5 gigs and has a good processor. So the speed is great it loads things fast downloads fine everyday usage stuff it works fine. When I try to play youtube videos the video is very slow and choppy and when i try to play like Minecraft it is impossible to play. When i play installed games like Flatout 2 and Need For Underground 2 (outdated games i know but still demanding) it plays them totally fine smooth no skips or lag. So i am wondering if it has a problem with Java and or Flashplayer since it seems to be slow when in needs to use that software?? The computer runs on Windows XP SP2 FYI. The sad thing is, is i have a Acer Aspire One netbook with significantly less everything thing then the tower does and it runs things much better then the tower so IDK??? What should I try? What should I do? any suggestions would be helpful!! Thanks