This error usually occurs when you have set up Visual Studio to debug an existing web application running in IIS rather than the built in ASP.NET debug web server.
IIS by default listens for web requests on port 80. In this case, another application is already listening for requests on port 80. Typically, the offending application is Skype or Apache which by default takes over listening on ports 80 and 443 when installed.
If you do development in Apache too go to httpd.conf and find for below lines:
#Listen 188.8.131.52:80 Listen 80 ServerName localhost:80
Replace the port with some other port like 81 or 8080. Restart IIS and you are good to go.
If you think Skype is culprit do the following:
Skype -> Tools -> Options -> Advanced -> Connection:
Uncheck “Use port 80 and 443 as alternatives for incoming connections”
This will free those ports from skype; again Restart IIS and you are good to go!