Seems I have more trouble with getting MySQL running with Apache and PHP5 than pretty much anything else I do. So to save myself — and hopefully someone else — much fruitless Googling and Apache restarting, here’s what I found.
If you have configured Apache and PHP5 on Windows XP using the outstanding howto Core PHP Apache 2, PHP 4 & PHP 5 on Windows XP, and can’t load the MySQL module, copy libmysql.dll to either your windows or windows/system32 directory, and copy both libmysql.dll and php5ts.dll to your /ext directory (off your PHP install).