That's because you made the mistake of installing XP after 7 is already installed. XP predates 7, and so it doesn't know how to set up a dual boot.
Now, you get to learn how to restore the Win7 boot loader, by spending some quality time with your favorite search engine.
In my pc I got windows 7, yesterday I made another partition and installed windows XP on it. But now my pc only boots on win XP and I don`t know how to boot on win 7 that is the OS that I want to use the most. When I turn the pc on It does not give me the option to choose what operating system I want to use, anyone can help?

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