^What do you mean you have to do it manually every time?
Another way to change XP to Vista or your own customized theme is using Neowin UXTheme Patcher and downloading themes from DeviantArt. It doesn`t lag at all because it`s not running a program to change your theme.
For more information, here`s a thread on this topic:
It also has info about the XP SP3 patcher if you have SP3.