Rainmeter is awesome. Everything is changeable through some very easy .ini coding
I even made my own skin one time. The only thing that sucks about it is that it takes a while to learn all the terms and what stuff is. (Difference between 'themes' and 'skins', etc)
I stopped using Rainmeter only because I basically never saw it (I always had a window open over top so it wasn't really doing anything), and it used up resources that were pointless to waste.
And I don't see any reason not to get a Windows theme. They're super easy to install, and they can really bring back some intrest in the sexyness of Windows
http://browse.deviantart.com/customization/skins/windows7/visualstyle/
(Sort by "Popular" in "All Time" for best results)