Summary
At this point, you have a complete development environment set up on your Windows XP machine. Whether it's a laptop that isn't always connected to the Internet or a desktop you're on 8 to 10 hours a day, you won't have to connect to your ISP just to test out simple PHP scripts. With the latest version of Apache's Web server, PHP 5, and MySQL, you're ready to program and deploy -- all on your local machine. In fact, this is the exact setup I run on my laptop -- and I set up a new laptop using this tutorial.
You are now familiar with configuration files and might even be willing to tinker a bit with your personal setup. You might want to create your own virtual hosts for Apache, run multiple versions of PHP for backwards compatibility, or even generate several instances of MySQL to break out your data. Whatever specific configuration you use, I hope you've seen that you don't have to mess around with your ISP to build and test your applications, and you certainly don't need to buy and maintain a Linux server just for that purpose. While you can find plenty of good uses for Linux, Mac OS X, and heavy-duty operating systems like Solaris, using them purely on a staging machine for simple scripting applications sure isn't one of them.
However you use this tutorial, I hope it takes your development to the next level -- whether you're connected to the Internet or not. Enjoy!

