Something weird with my firewall? i donno what it is or how its happening but its getting annoying!

Details: Intrusion: Invalid Source IP Address.
Intruder: 255.255.255.255.
Risk Level: Medium.
Source IP address: 255.255.255.255.This IP address is invalid.
Destination IP address: MATT-1096F7EBA1(192.168.1.100).
Protocol: UDP.

i dont know where its coming from as 255.255.255.255 is usually a subnet? and no computers on our network use this as a subnet. also all computers seem to be virus free? its only started in the last few days or so and i get it about ever hour to half hour! and well when it happens it keeps comin up with a gay alert! - ive turned off alerts for it but i would like to know how to stop it if anyone has any idea on what is doing it?

what really gets me is its an incoming alert from an invalid ip?