OK, that makes your intentions clearer. There are three issues I see, and one not-so-clear question.
Question: on what machine do you wish to boot and run from the clone on the external USB drive? From one of the computers with Vista or Win 7 on it? Or, from the laptop that originally had the old small drive?
1. Booting from an external HDD via a USB connection requires that the host computer's BIOS allows you to do this. Some BIOS's do, and some do not. You will have to check whether your does allow you to set a connected USB drive as a boot device in Boot Sequence Priority.
2. If I understand correctly, the old small HDD was in the laptop machine before. Now I'm not clear, but I THINK you mean it is still in that machine, and you just want to use its software and Win XP OS on another machine. Anyway, you have made a clone copy of that old HDD to the new one, and now the new one is in an external drive that connects to a host via USB.
3. Here's where the question and complexity come in. IF you simply want to boot the laptop from the external unit that does not seem to make sense - the old HDD that already has Win XP on it is there already, so why boot from the external unit? That is why I asked for clarification. However, if I assume you want to use that clone in the external to boot another machine, there is a real problem (even assuming you can get it to boot from a USB drive). The issue is that the Win XP that is installed on the old drive in the laptop is customized with drivers for all the devices in that machine. But it does not have the drivers for all the devices in a different machine. Hence, usually the "other" machine cannot actually boot from that copy of Win XP. There are ways to deal with this that often succeed, but not always. If this is what you want to do and if you can verify that the machines CAN boot from an external USB unit, post here and we can advise on trying to make it work.
There's another major issue. Using ONE copy of Win XP from one machine to boot and run other machines violates the copyright on the software. The license only allows that copy to be installed and .used on ONE machine