if your games are running poorly on low graphic settings and you have a fairly decent computer. chances are your playing with a polluted disease ridden under cared for louse of a machine. If you are not exactly computer savvy there are a few tricks to get a better performance out of your rig.
- check your Task manager. see how many programs are running in the background. see what your CPU is wasting processing power on.
Things like malware, viruses, Trojans will be using your computer resources slowing it down. and if you don’t pay attention to such things, your computer is gunna get sluggish.
^ above is usually a big problem if you…
A) browse for torrents 
B) browse for porn 
C) let others use your computer 
BUT! malicious things aside, programs you installed yourself that you don’t close will slow you down, internet explorer, active virus protection, Steam and Steam-like copycats (close all but the one you are currently using) torrent programs, apps, and whatever other useless junk people like to install on their rigs.
then there is generally the plethora of useless active crap that windows has running in the back background. Your gunna need to search for that based on your OS. Not a big deal if you have a good CPU, but if your trying to scrape every bit of speed from your cpu, its usually a good idea to turn off a lot of the useless extras that are installed in windows. and there are usually lots of articles that will tell you what each thing does and if you really need it.
Best thing you can do is research all the processes in your Task manager and see which ones you can END.
of course this is something most of us hardcore gamers will do just to get those extra 2-3 FPS 