I work on a Joomla CMS installation at work and have worked with Joomla, Drupal and Wordpress for a few years.<p>I'm comfortable with Apache/PHP/MySQL but don't have much experience with asp.net/IIS or indeed a Windows platform CMS.<p>Who should I look at first? Umbraco looks interesting.
Well, since PHP will work with IIS (not to mention that Apache and MySQL both work on Windows, negating the need for IIS at all), is there a reason to switch? Joomla, Drupal and Wordpress all work well on Windows servers.<p>That said, Umbraco is interesting if you need something in the .net space. My few remaining Windows servers all run PHP through IIS or Apache, so I can't really recommend anything else.