My PC says my other PC is port scanning me

Hi, I have a mac book using using Norton from COmcast and a Windows PC using Norton from COmcast as well.

 

Everytime I turn on the Mac I get a message that a vulnerability was blocked.  From address is the win PC's IP.

Name is Portscan

Application is m/mach_kernel

 

Why is my Win PC portscanning and how can i fix this?

 

Please help....

 

Thanks