The short answer is "you can't," especially because you are posting under Windows XP and some recent games won't run under XP.
I'm not sure if you are asking about OS compatibility or hardware resources; some hardware will simply be too weak to play some games.
And I sincerely hope that you are not asking how to get games that you haven't paid for to play; that is strictly against forum policy.
Can you give a couple of examples of games that won't play on your PC and what happens when you try to play them, as well as your PC specs?