Windows 7: Home network not working with my Netbook

I am having problems connecting my Netbook computer to my home network which started when I upgraded our two desktop computers to the retail versions of Windows 7. Previously, when running the pre-release version of Windows 7 on both computers, all was OK.

The network comprises a Virgin cable modem connected to a Netgear WGT624v3 wireless router to which is connected 3 computers.
My Windows 7 64 bit Desktop - ethernet cable connection
My wife's Windows 7 32 bit Desktop - ethernet cable connected
My Samsung Windows XP Netbook - wireless connection

The current situation is that all 3 computers can access the internet without any problems - thus indicating that the wireless connection to the Netbook is OK.

The two Desktop computers are able to share their files across the network via both Windows 7 Homegroup and via the Windows network.
My wife's computer is able to access the USB printers connected to my computer.
However - neither can see the Netbook and the Netbook is not able to see them on the network map. Furthermore, the Netbook cannot access the USB printers connected to my Desktop computer.

I have made sure that the computers each have different names, are using the same workgroup name, that file sharing is set up and the wireless WAP key is correct - to no avail.

If I connect the Netbook to the router by ethernet cable - both desktop computers can now see the Netbook and can access (read and write) its shared files. However, the Netbook cannot see the desktop computers and share their files - they do not appear on its network map.

My problem started after I installed the retail versions of Windows 7 on both Desktop computers - which of course resulted in the network being re-established semi automatically by Windows 7.

I am baffled - but have an ominous feeling I have overlooked something very basic......but what?

I need to add that I my computer knowledge is rather limited - but I am capable of most basic tasks and following the various wizards etc offered by Windows. I would therefore appreciate suggestions in relatively simple terms.