I like to use the classic theme and I go into advance properties and adjust for beet performance. This way, your computer will be much faster!
I have a windows 7 skin on my XP, even though classic will make it run more efficient, I just don't like looking at it...
I have the default theme on XP, with some of the graphics turned off to increase performing.. I've also played around with Administrative Tools -> Services to get rid of anything unnecessary.