Windows XP, for me at least, is the best operating system Microsoft has ever developed. It is stable, fast and only requires a small amount of resources to function properly. This is, of course, the result of the fact that Microsoft used a code they have used to develop their previous operating systems with, making it easier for hardware manufacturers to develop proper XP drivers. There is where the strength of this operating system lies in.
XP drivers are released regularly even today, which means the developers are still aware that many of us use this operating system. This is a very good thing, because we have talked about how important it is to always update your drivers. The idea is very simple. They managed to squeeze the Windows 7 and XP versions in the same package, which is very nice, but I think this sort of spoils the performance the drivers could have provided with on the long run if they developed them separately.
In any case, if you have to choose between XP and Vista, choose XP because you get better drivers, better performance and stability and even better updates for the system and all this at a significantly lower price.
Finally, XP drivers seem to be very simple to adjust to your likings, especially for motherboards and graphics cards. A lot of people have trouble setting their computer just so and getting the best performance from their hardware components.
With XP drivers this is really simple and you can set them up properly without too much hassle. You can also update the XP drivers easily and your setting will not be changed. Remember that you need to update your drivers as often as possible, and your computer will run as smooth as it should.