I have IIS installed on my windows 7 which uses default port 80 and can be accessed at
You have made XAMPP listen on Port 8081 rather thank 80 meaning that
needs to be
Without a port number in a URL you are just referring to the default port (I.e. port 80 for http)
Update: This is actually possible, however IIS and Apache (XAMPP) need to use different IP addresses and only bind to port 80 on there own IP address. http://www.iislogs.com/steveschofield/iis7-post-44-iis7-and-apache-on-the-same-machine
If you can't touch IIS then the only alternative would be to use a proxy script to show a site on IIS which would proxy to 8081.