open the run dialog (windows key + R) and type MSConfig.
Under the boot tab, see if there is one or 2 OSes listed. If there is only one, then the boot files are on the other HD. I think they'll end up on the XP hard drive.
You'll want to delete the boot files from it before you reformat the drive.
You can always just unplug the drive w/ Win7 and see if XP still loads nice, if it does than you can go ahead w/ whatever u want.
Why are you getting rid of Win7?