The Win32Service extension for PHP 5.3 can be found at http://downloads.php.net/pierre/
[EDIT] 2011-05-19 RQuadling : The version that is available on Pierre's site is an old version. At the moment, the only way to get this extension that matches the current documentation is to build your own.
Information for installing this PECL extension may be found in the manual chapter titled Installation of PECL extensions. Additional information such as new releases, downloads, source files, maintainer information, and a CHANGELOG, can be located here: » http://pecl.php.net/package/win32service