The first problem is you're in the Linux section and Linux has nothing to do with IIS or Windows 2003. It's going to be hard to find support for that in this area.
Running two web servers simotaneously on the same server just isn't a good idea, especially on a production server. I would highly recommend running XAMPP on another PC, develop your website, then copy the xampp folder over when you're ready to go live with IIS disabled or removed. Understand XAMPP doesn't actually "install" anything. It just decompresses the files into the directory xampp, then usually puts a shortcut on your desktop as well as add start menu items. But most importantly, it can be ran from anywhere.
There is a note on the apachefriends.org site about how to fix it if you change it's directory, since it does make direct references to the installations paths in the settings files.
My advise to you if you're running a public server and wanting to run Apache would be to look into converting your server over to Linux. I personally believe this is by far the best option for a public web server due to the improved security structure of Linux over Windows just from a default configuration. I initially had my public server running Windows and XAMPP and once I looked into Linux, I switched over to Ubuntu and used the Synaptic Package Manager to install Apache/PHP/MySQL instead of XAMPP. XAMPP is great for developmental enviroments and CAN be configured for production use, but isn't designed for that.
The good news is you can still develop your site using XAMPP, then transfer the files over to a production environment when it's finished.
If you want to know more about Linux and how to set up XAMPP / Apache on it, check out my website in my signature.
For great video and written tutorials and guides on creating your own web server and installing things such as forums, blogs, etc., visit http://myownhomeserver.com