This program will show you all currently used ports and the applications which use that ports.
You may get the program here: http://www.foundstone.com/us/resources/termsofuse.asp?file=fport.zip
After a short search on the internet I found out that I had the same problem like many people often too: skype is blocking the port for the apache. I thougt that it uses a "very high" port, that's right but there is also the option that it uses the ports 80 and 443 as alternative ports for incoming connections. So I turned that option off, did a new start of skype and it worked.