Hope this isn't a duplicate question. I have a new computer with Windows 7 64-bit installed as the OS. Can I boot into Windows XP from a different hard drive connected via a USB port? If so, how?
Frankly, I wouldn't want to, nor would I recommend it - USB ports are orders of magnitude slower than SATA ports.
It can be done (though it's unsupported), but I would suggest you're better off getting a Caddy system and just swapping SATA hard drives. Better still, run Virtual PC or VMWare and XP in a virtual machine (if you get/upgrade to Windows 7 Ultimate or even just business, it comes with XP Mode - a virtual PC install of XP that puts installed programs in the Windows 7 start menu).