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Skype uses port 80 by default. If it gets in first it prevents Apache using port 80. Completely exit Skype (on a Windows 7 machine you need to go to the taskkbar taskbar and right click/quit the Skype icon) and then start the Apache service. You can then restart Skype and it will use another port. Alternatively force Skype to not use port 80 and 443: In Skype | options | advanced | connection untick the "use port 80 and 443 as alternatives for incoming connections". This should also solve the problem.
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