CustoPack Tools : Customize Windows XP, Vista, 7 Image credit of Clubic.com
Just discovered that on Clubic.com
I think it's worth a try because you don't have to change system files by yourself
On top of that, the website offers a gallery for theme packs. Today, it's limited to 6 themes for Windows 7 though. But, I guess you can make your own easily with that software.
System supported :
Windows 7 (except for Starter)
Be sure to desactivate the Ask Toolbar during install and, in the options, desactivate "Ask me often to install the Ask toolbar".