typically Port 80 is the default Port only for Websever (like Apache, IIS aso.).
Other Progams although use this Port, such as Skype.
If you are running XAMPP on a Windows Mashine, make sure that the Microsoft Webserver IIS doesn't run.
In Skype you have to deactivate the setting that says that Skype is listening on Port 80 and 443. This setting you'll find in the Options-PopUp in the Category Connections (i don't know how it's called in the english version of Skype).
That's all i can say. Maybe someoneelse has another idea.
Kai aka Knight1
Sorry for my bad English.