Edit your apache configuration file (by default, APACHEDIR/conf/httpd.conf)
Look for a line that looks like
AddType application/x-httpd-php .php
and edit appropriately. Note you can have more than one entry here, just make sure each one is separated by a space; i.e.
AddType application/x-httpd-php .php .phtml .php3 .inc
is also a valid configuration
HTH,
AC