Windows XP Install help?

I'm using an iso image of Windows XP SP 2, and I've installed through iso before, on the same computer. Currently, I have Windows Vista, and i've become tired of the constant slowness (the computer I'm using is made for XP). I've tried two options; unzipping the iso and starting the SETUP straight from there (which is what usually works), and also mounting the iso and restarting the computer. When I use the first method, I press install Windows XP and it says "This program was blocked due to compatibility issues". And the second method, nothing happens. Can someone help me?