The Neowin UXTheme Multi Patcher allows you to use any 3rd party theme on Windows XP/SP1 or Windows Server 2003 (with Themes enabled) by patching the uxtheme.dll (dynamic link library). The process can be reversed but it should be noted that patching the themes service will break Windows File Protection on that single file. If you are prompted to "insert your Windows CD" and you intend to patch Windows to allow 3rd party themes you should click cancel and reboot.

What's new in version 1.01
Faster and more stable patcher
Fixed previous problems while patching (no confirm for 100% fixed)
Updated patcher supports Windows XP visual style
Supports Windows XP SP0 (without SP1)

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source: Neowin