1) Microsoft Web Platform Installer 3.0 (Web PI) is a free tool that allows you to easily install and run the most popular free web applications for blogging like Wordpress, dasBlog, and BlogEngine.NET with the built-in Windows Web Application Gallery. You can download this from: Microsoft Web Platform Installer 3.0 Web site. You can feature links directly to the webinstallers on this page and offer a tutorial for users on what to put in for settings to setup the blog under your servers.
A web-based installer creates a serious security risk on shared hosts because of file ownership. Anything your installer can write to is wide open to other accounts on the same server. You should drop the privileges after the installation.
2) Windows Installer XML (WiX) is a toolset that builds Windows installation packages from XML source code. The toolset supports a command line environment that developers may integrate into their build processes to build MSI and MSM setup packages. There's a tutorial available for customizing and setting up WiX here: customizing and setting up WiX and you can download WiX here
3) You can always ask your web hosting provider for the scripts they use to do the web installation of blogs from their control panel and then feature that script on your site.