To install Tiki via the Web Platform Installer (WebPI), you need a Windows version where IIS 7 is available (Windows 7, Windows Server 2008, most Windows Vista editions). The install may be slightly smoother using Internet Explorer. It will not work on Windows Server 2003 and Win XP.
At first, WebPI installs the dependencies for Tiki to be able to work (PHP, MySQL), then, it sends you over to the Tiki installer (tiki-install.php) where you follow the usual steps.
When you are in WebPI, make sure to make a note of your database information (user & password) that you set as the Tiki installer will need this.
If you are upgrading, you'll be required to enter the DB password. If you forgot it, you can look in C:\inetpub\wwwroot\tiki\db\local.php to retrieve it
When it comes to sending mail from PHP, the following configuration of Sendmail may be necessary: http://forums.iis.net/t/1165177.aspx. Alternatively, when the installation finishes, you can go to Admin > General and set up Tiki to send mails via SMTP instead (check settings given by your ISP / email provider).