Windows Vista/Windows 7 have prefetching, but it shouldn't be doing that. Do note that what Task Manager reports as RAM in use also refers to prefetched RAM. It should release said RAM if you run out and need more though. You also shouldn't be running out RAM just installing a game with 4GB (I never did). For reference, with 8GB, my used RAM in idle cases (after a fresh boot, or with Firefox and Winamp running) is often ~1.23GB (that's where it's sitting at now, and I just got done running The Sims 3, which can hog a decent amount of RAM), so it's not alot that should be in use if you're well equipped with RAM. Are you sure something you're running doesn't have a memory leak or something?
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