I have just realised that Apache has not been starting, so, I try to start it up from the Apache Monitor.

Apache Error – The requested operation has failed!

Looked in Event Viewer:
The Apache service named reported the following error:
>>> AH00451: no listening sockets available, shutting down.

Something has stolen the use of Port 80 and the usual suspect for this is Microsoft’s own webserver IIS which is run by the W3SVC service. So lets fire up Services and have a look.

And there it is, hiding under the Display name: World Wide Web Publishing Service.
Right click on service and select Properties:
Stop service and set Startup type: to Manual.

Now Apache should start – back in business