Invisible network PC

I'm trying to set up a very simple network, with two desktop PC's and a laptop connected via a Netgear wireless router. I've managed to get almost everything working, but the last piece of the puzzle is defeating me! That is, no matter what I do one of the desktop PC's remains 'invisible'.
Briefly, the two desktop PC connect with ethernet cables via the router; the laptop connects via wifi to the router. All connect to the internet via the router (and a cable modem). There is one printer which is attached to the 'invisible' desktop PC via USB. All the computers run Win XP.
Strangely, I can print from the laptop, indicating that the otherwise 'invisible' PC is accessible via the network.
One difference that may be significant is that the invisible PC was, and still is, connected to the internet via Virgin Media ISP and has already installed firewalls and anti-virus from Trend Micro; on the other two I use AVG.
I've looked at the Microsoft help website, which wasn't very helpful; I've tried turning off the firewalls, and some similar suggestions from MS, but all to no avail.
Perhaps some networking guru can point me in the right direction? If I've left out some vital bit of info then my apologies, but it's difficult to know what is crucially relevant.
Thanks.

I'm assuming that all your pc's are in the same workgroup? If not, they should be. File and printer sharing should be turned on all of them. Also, make sure network discovery is turned on.

http://windows.microsoft.com/en-US [...] me-network

http://windows.about.com/od/networ [...] indows.htm

The first link is for vista, but should work for XP.

The two links in my last post are in regards to network discovery. Maybe you didn't read far enough. This may be a vista/7 feature only as I can't find network discovery on my xp machine so file and print sharing may be all you need on xp.

If you list your OS's, it may help.

This link may help also.

http://www.tomshardware.com/forum/ [...] a-machines