Developing Websites with XAMPP is always neat and nice: Compact package, everything you need is there - and even having it portable on an USB Stick (in an TrueCrypt file ;)) will get you up and running on every Windows PC - where ever you are - in no time. But the problem with this solution comes with the "Host PC": There are a lot of tools nowdays which hijack and use Port 80 (your most beloved Webserver / Apache Standard Port) for their own needs. And so you are quickly on the point of "Apache terminated" in your XAMPP Windows - because of the blocked port. Finding out which program is blocking / using that port is easy, as XAMPP comes with an "Port-Check" Tool which can be used from the XAMPP Control Panel. Other than that, here is the TOP 4 of the most annoyning port blocking apps:

4.) Teamviewer (as commented by Teufelsauge)
A remote desktop tool which can be configured to run as background service, listening to port 80 and 443.

3.) Apache
Yeah, thats right. Some people got an old Apache setup running - set up years ago and forgotten in the background. Sounds stupid, but as most errors are - this is one of the most nasty ones. So check your memory before jumping to wrong conclusions.

2.) Microsoft SQL Server
It is an SQL Server! Not an Webserver! Why the heck would... Yeah, right: It DOES block port 80. Open services.msc from your "Run As" Dialog and look out for "Sql server reporting services(MSSQLSERVER)". Set it to disabled and stop it. Port 80 freed, hurray!

1.) Skype
To bypass corporate security standards and firewalls, Skype can use Port 80 and even Port 443 (SSL) as workaround. And it does that, by default. To disable that, start Skype and go to Tools -> Options - > Advanced -> Connection. Disable "Use port 80 and 443 as alternatives for incoming connections".