|You may need to connect the external drive to a different USB port.|
See response 3 in this:
External hard drives cannot get enough power (current) from one USB port.
3 1/2" drives in an external enclosure require an external power adapter be connected to the external case.
2 1/2" hard drives in an external enclosure require TWO cables with TWO USB connections on the computer end, one cable only needs two wires for 5v power, or the cable that needs only two wires can be connected to a an external power adapter meant for supplying up to 500ma at 5v, such as one with a USB port for powering an MP3 player.
If you want to be able to boot an operating system from a USB connected external drive, you must have the ability in the bios settings to be able to have a USB connected drive listed before any internal hard drive(s) in the boot order.
If the hard drive you copied had Windows installed on it when it was connected to a different mboard, if the mboard in the present laptop is more than a little different, XP will often not boot into Windows - what you typically see is the first bit of Windows graphics, then a black screen, blinking cursor top left, and nothing further happens.
Is that what you're seeing?
If you want to be able to use the copy of the existing Windows installation on the hard drive on the present laptop, if Windows was installed from a regular Windows CD on the other computer, you can run a Repair "install" procedure, instead of a regular Windows Setup, and not lose any of the data you added after Windows was installed.
However, I don't know if that option is available if the hard drive still had the original Dell software installation still on it, and you may need a regular Windows CD in order to run it in any case.
If you DO have that option to do a Repair "install", the SECOND time you are asked if you want the Repair Windows ("Repair your existing Windows installation", or similar), then you will need to do a few other things after Setup has finished - you will need to re-register some Microsoft Windows Update related files before Windows update will work properly, and you may need to re-install some Windows Updates
Many call the second Repair choice a Repair "install" of Windows, but I think it's more properly called a Repair Setup procedure, because what is does is run Setup again without deleting the contents of the partition Windows was installed on.
How to do an XP Repair Setup, step by step:
If the hard drive still had the original Dell software installation still on it, you will NOT be able to use a Recovery CD or a Recovery CD set made for the other computer model on the computer you have the hard in or connected to now, if they are not identical or in a small group of allowed models - the Recovery program will quit when it finds the mboard, or some other requirement, is different.
When you run the Repair "install" (Repair Setup) procedure, Setup may not find the SATA hard drive, depending on settings in your bios Setup. If that happens, look in the bios settings and set the SATA drive detection/controller to IDE compatible mode or similar, then run Setup again and Setup will find the SATA drive.