how you can choose the ammount of the harddrive that you allow windows to use

i got an intel mac,

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And i got windows xp professional installed on it using boot camp, i can play games and browse the web on both, but you know how you can choose the ammount of the harddrive that you allow windows to use to save stuff?
how do i change that, i know its able to be changed but i just cant figure out how,

please help me out with that

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