Plugin crashes! Physical Memory and CPU all used

"New" 2nd hand PC and the CPU and Physical memory crashes constantly, everything freezes for minutes, it even happens when I have my internet banking and excel open but its more frequent when i want to play games, how do i change the settings to stop this?

I have an Acer PC with Windows 7, 64 bit op system and a 2GB RAM, I am running firefox and have Adobe 15

Thanks Jeanine