a 100% full system drive causes everything to run slow .. either clean up a bunch of stuff .. uninstall a bunch of stuff .. or get a bigger or a secondary drive to install your games on or move your personal folders on to
Generally , you don't want to go over 80% full of your drives capacity. There are a lot of temp files created and what not .. as well as your Pagefile that your system will use in the event it runs out of physical memory .. ie .. you have 16GB of RAM installed and the game, program, whatever you are running wants/needs more than that ..then your OS will utilize your systems Pagefile that resides on your drives to compensate .. your system drives are of course waaaaay slower to access than your systems RAM .. and being that your drive is full . .then it has to do a bunch of shuffling around on top of accessing a slower medium to get what it needs to get done
...a big "ish" on my explanation of Pagefile stuff as I am no expert, But hey! ...that's what Google is for! .. or Bing! .. or Duck Duck Go ... or Yahoo! .. or ...