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).