For me, it turned out to be that "Web Deployment Agent Service" was taking port 80. As soon as it was disabled apache started right up. I've also come across quite a few saying some of the SQL Server services were blocking it.
EDIT:
Just thought I'd add that the new MSN(Skype) causes this as well, using port 80 by default. It can be changed in the settings:
Tools > Options > Advanced > Connection > Untick "Use Port 80 and 443" > Save
