If the system tray icon is flashing, the Personal Firewall is in Alert Mode because:
An attack has been recorded on your computer in the Security Log.
OR
You are missing a file or an application that is required by your System Administrator before you can log into the network. If a file or an application is missing, a message is probably displayed that lists the missing files or applications that you need to install before you can proceed.
To assess the status of the attack:
Double-click the System Tray Icon
If the reason for the flashing is an attack, the Security Log is automatically displayed. Otherwise, the icon stops flashing.
The system tray icon can tell you many other things, as described here: "Shortcut to the Software: the System Tray Icon"