Windows 7: Win7 boxes suddenly refuses to recognise XP boxes on the LAN

If I believe what I read about Windows 8, it's more infuriating than Windows 7. So far, though, Windows 7 provides enough frustration for a lifetime.



The requirement here is that four Windows boxes on this home LAN, two running XP and two running Windows 7, see each other's drives and folders with full control and without passwords. Effectively, they comprise a single integrated workspace.

Last year, after sustained fiddling that ought not to have been necessary, I succeeded in making the two existing XP boxes visible to two new Windows 7 boxes on the LAN, without passwords. The XP and Windows 7 boxes belong to a named workgroup. Sharing is correctly set up. I thought this issue was settled.

This week, the two Windows 7 boxes, without any changes on my part, have started demanding a username and password for one of the XP boxes.

How do I persuade the Windows 7 boxes to restore access to that WinXP box!?