I'm getting a similar problem, after a reshuffle of computers on my network.
The network is based on a Netgear DG834G v2 modem/router, which is an excellent bit of kit that has given me no problems. Until recently, my HP Pavilion laptop was the main computer and had a wired USB connection to the HP PSC1210 all-in-one, which was shared with the network. This arrangement worked fine, and all 3 of the computers on the network could see each other, and the printer (so long as the laptop was on).
I now have a new office computer, a Dell 9150, to which the printer is now connected. Two of my laptops can see this computer and the printer, and life is hunky-dory for them. However, the HP laptop can't see the other computers or the printer, though the other computers can see it! It can, however, see and connect normally to the internet, via the Netgear router. On selecting 'show workgroup computers', the old HP has a think then announces: 'xxxxx network is not accessible. You might not have permission to use this network resource. Contact the administrator... (yawn!) etc.'
I have confirmed that the workgroup (including its name) is set up (and re-set up) identically on all the computers (all running XP SP2, the HP is XP Home, the 9150 is Media Centre, and the Vaio is XP Pro). I have tried with the HP's firewalls off and on (Windows and Norton Internet Worm Protection). File/printer sharing is on, and all computers have at least one folder set to share.
Any thoughts?