ok sir what do you mean i will lost the ability to load win 7?? example if i installed the xp then the win 7 will not boot??
Correct, after you install XP it will break the booting win 7. Don't worry, no data is lost or anything on win 7 it just can't boot at the moment. If you look at step 7-11, under "When Windows 7 is Installed First" on the page provided by WR2, it will explain, how to fix win 7 boot using EasyBCD.
Also if for some reason your XP install didn't work, you can grab a win 7 disc and run a repair and it will fix win 7 booting for you.