How do i deal with this alert message?

My girlfriend recently bought a new laptop which she is connecting to the internet via a modem. She has the trial version of Norton Antivirus.

Since connecting to the internet, she has been getting an alert pop-up saying that a "remote system is trying to access your computer".

It gives options to block, allow and all the usual options, but we are not sure what to do with this.

Is it something we can stop repeatedly popping up? Is it something innocent to do with her connecting to the net or is it something more sinister like someone trying to access her system?

We are'nt too knowledgable on these matters so any help would be most welcome.

Thank you,

Calv.