How far do you want to go with the look and feel? Because XP simply isn't capable of the full-on Vista "experience" UI wise. You can change the color scheme and make it look like Vista as far as static images go, but you won't get any of the Mac OS X inspired multimedia functions out of XP... Ever. Vista has a completely reworked UI subsystem that makes that all possible, and seeing is how it's about the only part of Vista really worth bothering for, Microsoft will never backport it.

If you're content with just making it look like Vista to the casual observer, then something like WindowsBlinds should probably do the trick, though there is a performance penalty to be paid for it.