The simple answer is No... the operating system wouldn't work or is very unlikely to work. There are several reasons for this but, primarily, it's because it's very unlikely that the hardware is identical.
There are several caveats to this. IF the USA PC uses standard hardware that was in the Windows XP hardware compatibility list at the time the OS on your PC was installed AND your hard disk is the same type (i.e. IDE or SATA) as the USA one then it's more than possible that Windows XP will be able to self-install the right hardware drivers (or enough for you to be able to use it temporarily).
Another thing to consider is Windows XP Product Activation. If the XP Pro on your PC is a retail version (i.e. not a Volume License version) and the hardware is considerably different (which it is almost guaranteed to be) then it will trigger XP's built-in Product Activation.
You would also need to be incredibly careful about storage and shipping of your hard disk (especially with any possible current restrictions about hand luggage for airline passengers). All it would take would be a careless baggage handler (yeah, I know this is unlikely <grin>) and you would be absolutely stuck... both in the USA and back home.
Lastly, who knows whether XP would revert graciously when you stuck the hard disk back in your home PC. In my opinion it's just not worth the risk.
I suggest you have a look at an alternative that may meet your needs. See
for information about a fairly recent type of USB drive that may allow you to take what you need with you.
I've been testing two SanDisk Cruzer models for months and have found it absolutely invaluable for carrying around my favourite stuff with me, including all my Favorites, online passwords, etc.
The tests have been so successful that I've just been asked to order 23 512Mb U3 drives at work for our roaming users and I'm getting 1Gb ones for my family as Xmas presents.
Hope this helps... and have a good holiday!