It is installed in windows Vista but the process is same like any other windows platform.
It is advisable to run IIS before installing PHP. You can download current version of PHP from php.net/downloads.php page. You can download with installer. For example you can download PHP 5.2.12 installer link. The link may vary depending on the version you are downloading.
Setup Wizard
You can start installing PHP like any other windows installation. You will get the setup wizard which will guide you through installation process.
Web Server Setup
On this screen you will be prompted to select the web server you wish to setup. Out of a list to select we have to select IIS ASAPI module.
Choose Items to Install
We will be asked to select features which we will be installing. Here you can expand each feature shown in screen shots. If you have a plan to use MySQL then you must install this feature. Similarly GD library supports you need to add if you are going to work in graphics. Based on your requirements you can go ahead and install the features you wish to have.
However the installation process has became easy with the windows installer. If this is done properly then you may not have to work with php.ini file to do the manual setup.