I can’t say enough about “Degunking Windows,” by Joli Ballew and Jeff Duntemann. Not only was I able to improve the performance of my XP machine, but I used some of the same techniques to get my wife’s old Windows Me machine running (mostly) like new again.
Some of the stuff is common sense things that you know you *should* be doing anyway, but having a nice list of steps to accomplish really helps get the ball rolling.
(In the interest of full disclosure, the company I work for distribrutes books for the publisher of this title, but I did buy this one myself for my own personal use.)