Desde PHP 5.x en adelante el soporte de Mysql no viene por defecto pre-compilado (las librerias) como así es en PHP 4.x ...
Por ende . .te hace falta dos cosas para poder usar las funcoines mysql_xxx() con Mysql:
1) Habilitar la extensión de PHP
extension=php_mysql.dll
(y el que el directorio extension_dir = apunte al directorio donde están esas php_xxx.dll
2) Mover la librería de Mysql de la versión que usas hacia tu directorio "windows/system" o directorio de sistema en general del S.O. Windows que uses
Todo esto lo menciona la documentación oficial de PHP:
Cita: Installation on Windows Systems
PHP 4
The PHP MySQL extension is compiled into PHP.
PHP 5+
MySQL is no longer enabled by default, so the php_mysql.dll DLL must be enabled inside of php.ini. Also, PHP needs access to the MySQL client library. A file named libmysql.dll is included in the Windows PHP distribution and in order for PHP to talk to MySQL this file needs to be available to the Windows systems PATH. See the FAQ titled "How do I add my PHP directory to the PATH on Windows" for information on how to do this. Although copying libmysql.dll to the Windows system directory also works (because the system directory is by default in the system's PATH), it's not recommended.
As with enabling any PHP extension (such as php_mysql.dll), the PHP directive extension_dir should be set to the directory where the PHP extensions are located. See also the Manual Windows Installation Instructions. An example extension_dir value for PHP 5 is c:\php\ext
Note: If when starting the web server an error similar to the following occurs: "Unable to load dynamic library './php_mysql.dll'", this is because php_mysql.dll and/or libmysql.dll cannot be found by the system.
www.php.net/mysql
Te recuerdo también que puedes usar las nuevas extensiones para Mysql 4.1.x en adelante:
www.php.net/mysqli
para sacer un mayor rendimiento a tu Mysql 4.1.x o superior desde PHP 5.x
Un saludo,