Windows does not allow you to install 3rd party themes. You have to patch some system files to be able to do so. UxStyle Core lets you install themes on Windows 7 & Vista easily.
UxStyle Core does not currently have a theme manager or even a UI. UxStyle Core is exactly what it sounds like – the absolutely bare minimum (core) needed to enable third-party theme usage. Nothing more, nothing less. Later in the beta, we’ll push out a “bundle”, containing both the core software and a theme manager developed by our close partners.
Install UxStyle Core. It runs as a service in background. Download some 3rd party themes. Give them a whirl, everything should just work.
You may want to check out Windows Theme Installer !