abled) just by patching the uxtheme.dll
It is a patch, but i is very easy to install, you only have to run the .exe file, UXTheme MultiPatcher detects the OS you are running and it will do the rest. Since the moment you install it, you will be able to download and install any msstyle theme, with no restrictions.
It supports Windows XP/SP1/SP2 and Windows Server 2003 SP1.