It sounds to me like the scanner you used is getting confused. UDP port scanning is very different to TCP scanning, and is pretty unreliable. With a UDP scan, you send a packet to the port, and if you get no reply it means the port is listening. If you get an unreachable message, it means the port isn't listening.

The problem here is that most firewalls silently drop the incoming packet, so the unreachable message is never sent back. This confuses the scanner into thinking the port is open, when in fact it's being protected by a firewall. So, if all but a few ports appear open, you can assume they're actually protected by a firewall, and the ports that are reported as closed aren't. The only problem with assuming this is that you can't actually tell what ports really are open, unless you trigger an event on the PC you're scanning that causes something to be sent back to you.