Recently, I had problems installing PHP with the Windows IIS server. I was installing PHP 5.2.1 on a Windows 98 machine running IIS 5.1. The PHP website offers the option of using a Windows installer or doing a manual installation using a ZIP file. My recommendation is to use the Zip-File. I had installed PHP using the Windows Installer and indeed everything was comfortable, went quickly and seemed to be OK. Yet, when I tried to run a PHP program I had many problems. Many times it worked but many times I got an Html-Error 500 in addition to many other errors. At times it the PHP program would work and then suddenly an error. It was one of those nasty bugs which is not directly reproduceable but occurs somewhat randomly. After much googling I installed PHP using the ZIP-file and the php.ini-recommeded file as a starting point. And it was like magic, no problems whatsoever. Again avoid the windows installer, do it manually.