Your port 80 is already in use by another process (IIS, Skype, Teamviewer, ...). Find this process and stop it before you start Apache.
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I take it you are using XAMPP and if so then download the new Control Panel Version 3 then extract only the xampp-control3.exe file from the zip file into the xampp folder and double click to run instead of the one included in the release.
The download link is in my signature.
Then look for and press the Netstat button and all will be revealed.
Also look in the \xampp\apache\logs\error.log file to see if it has any clues that might help.
Click on the Win-Services button in the CP3 and look for Apache2.2 entry.
If you find it means that at some stage you have selected to run Apache as a Windows service usually by using the tick in the Svc box in the old CP.
If you find a service then in your new panel click on the tick next to Apache and then answer the question. Then click in the tick again to remove the service.
You should then be able to start Apache using the Start button in CP3.