As Microsoft class Vista as a later version of windows, you can not install XP as an upgrade and therefore, you must do it as a fresh install. Insert your Cd for XP and boot to it. When you get to the window about where do you want to install, select the Vista installation and select format option.
Note:- You will lose all of your programs and data and you have to reinstall. make sure that you back up all the data you want to keep and ensure that you have all the drivers for your PC ready before you install Windows.
Before you start it would be a good time to make a slipstreamed Cd of your XP operating system with your PC's drivers, XP serial number, and a lot of updates including SP3 already on your CD. It will save a lot of time when you do install or repair Windows XP.
"one more thing if you want to install both of two windows then you can install it two diffrent drvie."
one more thing if you want to install both of two windows then you can install it on two different PC's.
Or maybe, as you said, you want to remove Vista and use XP, maybe you don't want to keep Vista, Maybe you only want XP, then you will not need two different drives or PC's. I might be wrong but that is what you said.