Because most of them probably contain malware and such. Only reply if you know for certain that they are safe sites. Thanks. I'm getting sick of these default themes.
The Vista Transformation Pack is pretty cool. It make XP look almost exactly like Vista. It even comes with transparent title bars and a clone of Flip 3D now.
Normally, you can't install unofficial .msstyle themes in Windows XP. However, you can install unsigned visual styles by applying the uxtheme.dll patch to your system. Visual styles can be downloaded from deviantart if you navigate to: Customization > Skins & Themes > Windows Utilities > Visual Styles .