If installing the Cassini Personal Server on a PC you will often get the “Cassini managed web server failed to start listening to port 80. Possible conflict with another web server on the same port.” error.
You of course think this is a firewall, other web server or anti virus port blocker problems
IT IS NOT!
Ok it might be those problems as well but usually it is that you need to run
gacutil /i c:\cassini\cassini.dll
or just drag a copy of the cassini.dll into the GAC (C:\Windows\Assembly)
Shame the installer does not do this.