I can't say that I've ever come across a Win 7 machine where IIS was installed and running by default, but certainly that could still be an issue. You may want to do a quick Google search on disabling or removing IIS in Win 7 to make sure all the components are completely inactive or uninstalled. You definitely shouldn't have two web servers running on port 80 at the same time.
Try removing IIS completely if you have no need for it. Completely uninstall XAMPP, and then do a little clean up to make sure you get anything it may have left behind. Try a reinstall of XAMPP, paying particular attention to any installation dialogs and options along the way (install as a service, open firewall port, etc..).
You might also want to take a look at some of the guides and tutorials out there for XAMPP. Maybe something you find will lead to a clue about why it's giving you a tough time. XAMPP "how to" windows 7
All things being equal, that should give you a working installation where you can test apache, mysql, and php to make sure they are all operating correctly before setting up WordPress.
If you still see no success with XAMPP, you might try WampServer. I've had very good luck with it on Win 7. You just need to install MS visual C 2010 redistributable first on Win 7. Just make sure any packages you select are for your platform. (64bit).