If you get this error: "Service Unavailable" after installing PHP to a Windows XP x64 Pro,
you may want to try checking the ISAPI Filters in your IIS.
Make sure your a running the 32-bit version of the Microsoft.NET\Framework\v2.0.50727\aspnet_filter.dll.
If you are running the 64-bit version, you will notice that liested item in the ISAPI Filters has a red arrow pointing down. This means that the service failed to load. Delete this item from the list and add a new one. Make sure you use the one located here: %Windows%/Microsoft.NET\Framework\v2.*\aspnet_filter.dll.
Also make sure the permissions to your PHP folder have "NETWORK SERVICE", "IUSR_*", "SYSTEM" included.