Opening local files contacts router through NetBIOS

I'm networking an XP desktop with a 2000 laptop using simple file sharing
(with NetBIOS). The laptop is used to access shares on the desktop. The file
sharing works fine.

However, on the desktop, whenever I open any 'normal' local file on the hard
disk, there is a five second delay. Looking at my firewall, it appears this
time is spent in sending 1404 bytes to my router, and receiving 1404 bytes
back, through the NetBIOS_NS port. It doesn't seem to matter whether or not
the local files are being shared.

My question is: why does the desktop need to query the router when opening a
file off it's own hard disk? The problem disappears if I disable NetBIOS
(and the laptop seems unaffected by the problem).

Thank you for any help

Matt.