Installation troubles with MySQL

I’m new to this whole webdesign thing, but WordPress looks to be exactly the blog software I want/need. I followed the installation instructions to the letter (I think). And yet, when I try to open the install.php (the last step), all I get is this error message:

Your PHP installation appears to be missing the MySQL which is required for WordPress.

PHP is version 5.0.something, MySQL is version 4.1.9, Apache2 server.

So what do I do now?