Hi,
In order to modify my desktop, I'm using a sort of program which modify the file Uxtheme.dll in c:\windows\system32 (Win Xp), and allows me to load some themes I added in the folder c:\windows\ressources\themes.
It runs very well, working like a normal theme, doesn't use more ram, and despite I have been using it for around 1 year, I never had any bug or problem with windows.
Budbud.