There are some good ones here: winmodify.net
However, I am currently using this one: VistaVG Black Theme for Windows XP
Be warned: in order to use visual styles not licensed by Microsoft, you have to patch the uxtheme.dll file; the VistaVG Black page has a link to the file. If you don't want to muck around in your registry, you could try this licensed visual style, Royale Noir
; it doesn't do a full makeover of XP like the other themes, but its less risky than the others.