I'm not sure whether this applies only to Windows Firewall, which I'm using, or any firewall.
Recently did a firewall test at HackerWatch.
With the open ports scan, managed a full stealth rating.
Did the same test 10 minutes ago with Skype active, but minimized, waiting for my son to come on line, something we do very regularly.
HackerWatch port scan showed ports 80 and 443 as red. In other words, open to the world and awaiting connections.
It appears that by default Skype uses port 13023 for incoming connections, but opens ports 80 and 443 as alternatives.
I've stuck this screen shot in as it seems the easiest way to show where this checkbox is, and as you can see, unchecking this option gives full stealth again in HackerWatch portscan test.
A lot of you guys who use Skype may already be aware of this, but if not, I'd check your prefs.
Can any of you knowledgeable guys tell me if that is as bad a security risk as it appears?