The MSSQL extension is no longer present in the 5.3.0 release, but this fact is not mentioned on this page. Perhaps that should be added.
It seems that the recommended solution is to use "SQL Server Driver for PHP" by Microsoft (http://www.microsoft.com/sqlserver/2005/en/us/php-driver.aspx). However, it is reported that the current release (v1.0) does not support PHP 5.3.0 (see http://forums.iis.net/t/1158918.aspx). Apparently, v1.1 will support PHP 5.3, but it is currently in CTP release.