Why is my computer so slow?

HELP!! So my problem is that the computer is running so slow like when i open and watch a video for a while its would freeze or hang as if its struggling to work. this happens every time i use it and some times its just irritating. some points that you should know is that this computer is over 3 y/o and it's had a couple of formatting tone to it. we install and uninstall games most of the time and sometimes i just turn off the AVR because it hangs when i try to shut it down. also there's a video card that is installed help me guys, what should i do? do i need to buy another desktop?