While Windows XP's default themes are pretty good, they are of course, a little limited - seeing as how there are only three of them. XP does have a theme format, allowing for user-created themes; however, by default, XP only recognizes themes made by Microsoft (which is a bit constrictive). However, the free PatchXP modifies the uxtheme.dll file on your system, allowing you to download any use any user-made theme. (Of course, a fair few user-made themes are much worse compared to the default MS ones!)