You are not satisfied with the themes for XP and Vista? Why not make your own theme then…

DeSpize gives you some links to get started with making themes.

Windows Vista

There is a beginner tutorial at JoeJoe available as PDF in English and German.
http://www.joejoe.org/forum/index.php?showtopic=10445

Windows XP

For the XP users, GFXCanvas has great and comprehensive guide.
http://www.gfxcanvas.com/home/Tutorials/p2000_articleid/2
http://www.gfxcanvas.com/home/Tutorials/p2000_articleid/3

If you know some tutorials related to making custom themes, please leave a link in the comment.