I Assume you will be dumping IIS (and using apache instead, of course).
Get php win32 binary from http://uk.php.net/get/php-4.3.4-Win32.zip/from/a/mirror
or http://www.php.net/downloads.php
(get the zip package is my advice, not the installer)
unzip it on to your XP Pro box into c:\php
copy php.ini-dist to php.ini
edit apache httpd.conf and set up for cgi (see the install notes with php, but basically do following with apache)
ScriptAlias /php/ "c:/php4/"
(tell is where php lives)
AddType application/x-httpd-php .php3
AddType application/x-httpd-php .php
(tell it what type of thing a .php file is)
Action application/x-httpd-php "/php/php.exe"
(tell it what to do with this type ie run php.exe in (scriptaliesed) php location /php/ which expands to C:/php4)
HTH
WaveyDavey