Firewall blocks Prot 529 UDP?

I noticed a few days ago that my notebook and desktop stopped talking to
each other. XP Pro on both. Both connected to a Netgear RP614 Router.

I see on the traffic log for my firewall (Sygate Personal Firewall Pro 5.5)
that traffic wanting to go through port 520 (Protocol UDP) is being blocked
both to and from the router and the notebook. This is so even though
there's a rule for my firewall to allow all traffic to and from
192.168.0.xxx.

Any ideas what's wrong? And anyway, what's port 520 (English please!)

Many thanks.
Ken