Go to http://www.php.net/downloads.php and download the 'PHP 5.x.x zip package' if you don't allready have it. This package contains the GD-Library for windows. Find php_gd2.dll (or php_gd.dll if you need gif-support, but you will have less good image quality) and put it in your php folder (Usual c:\php). Find in the php.ini in your windows folder:
;extension=php_gd.dll
;extension=php_gd2.dll
Uncomment extension="php_gd2".dll by deleting the ';' in front of it.