File Sharing Across A Home Network - The Network Path Was Not Found"

I'm having difficulty trying to get my new Windows 7 pc set up to share files across my existing home (wired) network which consists of 1 desktop pc running WinXP and 1 laptop running WinXP.I want the 'C' drive on each of the 3 computers to be shared. I've had the 2 Windows XP computers set up and working like this for several years without a problem but I can't seem to get the 'C' drive on the Windows 7 computer to do the same.All 3 computers have the same workgroup name and none of them require passwords to log on to Windows. There are no problems with the firewalls on any of the machines.On the Windows 7 PC when I right click on the 'C' drive and select the 'Sharing' tab, I have set this up to be shared and when I click on the 'Advanced Sharing' button there is a tick in the 'Share this folder' box, the 'Share name' is 'C'. If I then click on the 'Permissions' button, this shows a 'Group or user name' 'Everyone' and this group has Full Control, Change and Read boxes ticked. As far as I can see there is nothing more I can do.

However from my Windows XP computer, when I go to My Network Places and double click on the icon for the Windows 7 'C' drive, I get the message "\Computername is not accessible. You might not have permission to use this network resource. Contact the administrator of this server to find out if you have access permissions.The network path was not found"I've subsequently set up a sub folder (of the Windows 7 'C' drive) for sharing by right clicking on the folder, choosing 'Share with' and then selecting 'Specific people'. I then set up a group called 'Everyone' with read/write permissions. Now I can navigate to this folder from my 2 WinXP computers.