if your computer should be running the game fine, you've probably been infected with something nasty. I would say run anti-spyware etc but they generally don't work. How old is your Windows install? if it's more than a year old then it will be full of bloatware, and things tend to linger.
Look at your system tray, if it has more than 10 icons, you are probably running too much crap in the background. If you have an old Windows install, I recommend a complete reinstall, and be careful what you install (Do not put Norton or McAfee on your computer, it will kill your performance considerably)
Another solution (if the games support it) is to install Linux.
Of course, it's highly possible that your PC is simply not up to task, in which case, have a look at the requirements of the games. More RAM often helps performance, apart from that the only thing you can really do is upgrade.
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