Easily install unofficial themes on Windows 7 with a free program
Kishan Bagaria has put together Windows Theme Installer for The Windows Club and it makes the process a whole lot easier.
Download the app, run it, check the theme box, and browse for your theme file. Press install theme, and the app does the rest for you -- including auto-patching any files that need it. In a flash (about 5 seconds) I had the awesome Clearscreen Sharp theme installed on my system. DeviantArt has loads more nice Windows 7 themes if you want additional options.
Word of warning: make sure you create a system restore point prior to running the app. It does have a built-in restore feature which will put back unpatched versions of the files it modifies, but it never hurts to have a backup of your own. Also make sure you're downloading your new themes from a trusted source.