If you wanty IIS to run concurrently with Apache, you will have to set XAMPP/Apache to listen on a different port. Two applications can't listen on the same port. The usual choice is 8080, BUT it can realistically be any open port.
You could run both services on port 80, but you would have to shut one or the other down each time.
in httpd.conf in the \apache\conf folder find this
# Listen: Allows you to bind Apache to specific IP addresses and/or
# ports, instead of the default. See also the <VirtualHost>
# Change this to Listen on specific IP addresses as shown below to
# prevent Apache from glomming onto all bound IP addresses.
Change the Listen value to your new port for Apache, save ther file and restart Apache.
i,e Listen 8080
When you want to access APache/XAMPP type in http://localhost:8080