But as this is Windows you need hardware that will boot off a USB Device and it will only work on the Hardware that it was installed upon as the drivers required for different hardware will not be present.
If you want to use this as a portable drive between different computers again it's possible but not highly recommended as the time wasted doing constant Repair Installs will be great. You may as well just do a Repair Install of XP onto the existing HDD in the computer.
How to replace the M'Board you will need to follow these directions to use on different hardware every time that you use it. Well on different hardware at least if you have several computers of the same type/model it can be useful them but not on different models.
http://tinyurl.com/j9u7
How to do a repair install of XP
http://tinyurl.com/g4bv8
Also something you need to keep in mind External HDD are not a great thing to use full time as they get hot very hot and are very prone to vibration damage. So if you use a HDD like this it is going to fail sooner rather than latter. But if you absolutely must do this you need to employ some active cooling for the HDD enclosure and the HDD inside it. Start off with a metal Enclosure for better cooling not a plastic one and then using some type of Fan have air constantly flowing over it. Ideally blowing on it or a Heatsink connected tot he enclosure is the best but this may not be suitable to be portable any longer and you need to make it Electrically Safe.
Col