I have just installed Ubuntu 10.04 on an new disk, and updated it.
The machine is on a home Windows network with 3 WinXP machines and a router, all wired ethernet.
Internet (Firefox) works OK through the router.

At first everything seemed fine, but soon I found I cannot see the Ubuntu machine from the WinXP machines, although the Ubuntu can see them (it was OK at first).
Also, the Ubuntu put itself onto a workgroup by itself called "Workgroup" (the Windows default), but the XP machines are in a group called "Group". "Workgroup" is unavailable from Windows, so it could be a Windows question, but it worked when I first installed the OS.
Any ideas please?

You need to install Samba in Ubuntu to allow sharing between Windows & Linux
https://help.ubuntu.com/community/SettingUpSamba

That should fix it..

Today things are better! Now I can see the Ubuntu from the WinXP machines!
I still would like to move its workgroup to be the same as the Win machines.

OK - sorted I think. The help info had me edit the smb.conf file to show the group name, and it seems OK now. I see there are several copies of the file on the system, and the /etc/samba/ one worked.
Thanks for the comments anyway!