A good way to change your theme is to use the UXTheme patcher..
Download Uxtheme Multi-patcher 6.0 - A patch that will change your uxtheme.dll on Windows XP - Softpedia
Then use deviant art
Browsing Visual Styles on deviantART
to find a theme.. it wont replace EVERYTHING but it will replace the majority of windows and it's all free..