Yes its possible however there is a good chance it wont work or you will encounter lots of issues.
When you installed Windows XP on your old machine windows installed all the chipset & controller drivers for your hardware like motherboard, cpu, video card etc.
When you boot this operating system from your laptop you will probably experience issues as all the hardware is different.
Why would you want to boot this external drive from your laptop? If its for data recovery / copying files there is a much easier ways