As you know, in order to keep their branding intact, Microsoft protects the Windows XP themes with a digital signature, verified each time the theme is installed or run. This is the reason why only a few themes are officially agreed by Microsoft and can be installed on your computer without much of a hassle.
But what about all the fan-developed themes? Well, UXTheme Patch for Windows XP SP2 provides just the solution for that. It replaces the uxtheme.dll file located in your Windows/System32 directory and removes the signature verifications, allowing you to use just about any third-party customized XP theme.
Before replacing the uxtheme.dll file, you must know that this application only works with Windows XP SP2 and prove quite useless with SP3. If you are running a SP3 operating system you should try UxTheme Patch for Windows XP SP3 instead.
Moreover, a few safety measures are recommended before replacing the system file. Be sure to make a backup for the original uxtheme.dll by either renaming it to uxtheme.dll.old or copying it to another location. That is highly advisable since some of our users encountered problems while trying to recover the old file after patching.
Make sure to click Cancel, in case Windows prompts you with a message letting you know that an original system file has been replaced. If you don't, the patch is immediately replaced with the initial uxtheme.dll.
That is about all you have to keep in mind. After you replace the original file, make sure to reboot and enjoy the endless possibilities the fan based themes have to offer you. Your operating system can now look like OSX if you find the right theme.
We tested it with Sentinel XP Theme and we were able to replace the start menu, buttons, windows, dialog boxes, scrolling bars, menus, icons or taskbar and got whole different look of XP with no trouble at all.