Are you running any file sharing application?
Do you have any shared folders?
Even if you don't have shared folders it is possible for people to interrogate your machine in order to find out if you do or not - some firewalls may see this as an attempted attack using your NetBIOS port.
To pin it down it would be helpful to know what port the 'incident' targetted and what transmision protocol it was using, amongst other things.
Pasting the log from the firewall is your best bet (the 'advanced' one, if it has such a thing).