I've always put everything (apache, mysql, and the php) folders under just C:\ and not inside the program folder.
What version of windows are you using? I've just installed all three of these things plus phpmyadmin on two Vista machines.
On my first try on the first Vista, php seemed to be working when I checked the phpinfo, but I couldn't get phpmyadmin to install correctly. Turns out, my php wasn't installed correctly with all of the right settings in the php.ini file.