XP won't run if you install it on a HDD then put it in a completely new system. It's gives each component in the system a few points. It checks the total points at time of install to the total at time of boot. If it's off by more than 7 (I think) it won't boot. This lets you change some things, like a NIC, CPU, or RAM, but not a lot.
Don't install anything on a USB drive. It'll default to USB 1.1 and take a VERY long time.