Windows 7 OS running amok

Hello,

So, I have two 250 GB harddrives, one with XP, and one that used to run W7 just fine. I had it set to allow me choice between the two upon booting up. Now, when the W7 HD is connected upon power on, it will take over, force me to use it, and strat trying to fix itself only to sit at either a blank, black screen or the blue screen W7 displays before login. I have the OS hard disk, and I want to simply reformat the W7 harddrive (there's nothing important on there) and reinstall the OS, but in order to get to a My Computer menu, I have to use XP, and in order for it to launch I have to connect the W7 HD after booting up, and then the computer no longer recognized the Harddrive at all. Please Help.