Not much to say really.
For those that totally don't have a clue:
These are themes that change the appearance of your windows XP operating system.
EDIT: From what I've heard the install is pretty much automated so if you can install any basic software on your machine you can probably install this too.
EDIT 2: You can't modify the theme much once installed.
But as any other modifications to default windows settings you should have a restore point to be on the safe side.
If you like them, just guy buy them at http://www.winmatrix.com/hyperdesk/.
That will give the developers more money to make new ones that you may probably like too.