If you already own Microsoft Plus! and don't feel you need the additional features that Desktop Themes offers you, you might like to try Theme Installer, a freeware application that lets you install and uninstall themes automatically.
Theme Installer automates (to some extent) the Process of installing Plus! Themes. There are thousands of different themes available now, and most of them need to be installed manually (copy these files to your Plus!/Themes directory, these to Windows/System, etc..) What's more, they have to be uninstalled manually too.
Theme Installer reads the .theme file for the theme and works out where everything needs to go, and copies it there automatically. It also adds an Uninstall entry to the Registry. This allows you to uninstall the theme and all associated files from the Add & Remove Software control panel.