My new computer runs so slowly!! Why?

On my old computer I would be able to have multiple windows open (windows that run the Adoble Flash Player,Youtube, Tumblr & even cam websites like TinyChat) and it would run smoothly... But this brand new computer can barely run 2 of those websites... HOW COME??

Norton AntiVirus was preinstalled and I haven't uninstalled it yet, but I definitely plan to (although apparently you can't get Microsoft Security Essentials on Windows 8 so I don't know what todo about that). I also installed Guild Wars 2 and that runs slowly but it's still more or less tolerable for what I'm interested in doing. Could uninstalling GW2 make my computer run the websites I want it to normally?? Like I just tried getting on a pretty full TinyChat room but it couldn't handle that window all by itself... It's so annoying because this is supposed to be a computer update, not a downgrade... I didn't think that GW2 would make the computer THIS bad.... Please help!!!