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If you're running on a system with limited memory (under 4GB), and keep a lot of programs open when you play games, you're losing a lot of performance to idle tasks.
Though RAM is pretty cheap these days and upgrades are easier than ever to install, it's still important to shut down any unnecessary programs and applications before booting up your game.
Even without playing anything, iTunes can take up to 700MB of memory, and the average web browser can take up a few hundred MB more per tab open. If it's not necessary to enjoying your game, hit CTRL+ALT+DEL and shut it down; your system will enjoy the extra breathing room.