A similar question has been asked on Server Fault.
Open %userprofile%\documents\iisexpress\config\applicationhost.config
file and
Find following entry (or similar entry) in applicationhost.config file and comment it or delete it.
<application fullPath="C:\Program Files\iis express\PHP\v5.2\php-cgi.exe" monitorChangesTo="php.ini" activityTimeout="600" requestTimeout="600" instanceMaxRequests="10000">
<environmentVariables>
<environmentVariable name="PHP_FCGI_MAX_REQUESTS" value="10000" />
<environmentVariable name="PHPRC" value="C:\Program Files\iis express\PHP\v5.2" />
</environmentVariables>
</application>
Find following entry in hanlders section and comment this as well or delete.
<add name="PHP52_via_FastCGI" path="*.php" verb="GET,HEAD,POST"
modules="FastCgiModule" scriptProcessor="C:\Program Files (x86)\iis
express\PHP\v5.2\php-cgi.exe" resourceType="Either" />
By default Web Platform Installer installs PHP to %programfiles%\iis express\php. so open %programfiles%\iis express\php\ folder and delete the php version folder that you no longer need (don't forget to remove relavant entries from applicationhost.config as mentioned in step 1 and 2 above)