Such is what it is. What is the word? "Flighty".
There are patches to firewire even in in SP2 from memory so I'm going to write you should use ETHERNET network cards which are used daily by billions.
I run Windows XP and network to my husband's machine. We use a firewire short cord and use the Network Home and Small Office Wizard to set up our network and it usually runs just fine. Every so often I appear to be on the LAN but I am not, do not show an icon for the internet connection. Nothing I do seems to fix the problem aside from running the Wizard again. It has created some 20-odd "Network Bridges". Although the numbers seem to be different on the two machines (ie Network Bridge 20 on one, Network bridge 23 on the other) it usually works ok. I suspect it is the Norton Antivirus shutting off the LAN at times. Is there some way to correct this without going through the Wizard? When I try to 'repair this connection' it usually tells me it has failed to do so. Is there some way of running things manually -- correcting the IP address by hand? And what causes this? Am I right in thinking it may be the Norton?