you might want to try running linux on it. linux tends to run well on old PCs and PHP is a Linux thing.
The Suse update is only a few dollars and contains everything you need. The full version comes with two manuals but they can be downloaded free in PDF format anyway.
The Redhat Linux bible is available in most books stores and ships with the operating system on a few CDs. It's a great introduction into the world of linux and setting up servers.
Both these solutions are very powerful and cheap. Running php on windows 2000 is possible if you install the php.ini files in IIS or download the windows binanaries for the apache web server.