We have a network in the office that has been humming along for over a year now with this basic setup. We share files off of a Windows 7 machine. Windows 7 machines can connect to these shares just fine, and for ages now Windows XP machines could access it.
All of a sudden, today the XP machines can not connect at all to these shared folders. All of the settings on the Windows 7 PC serving the files look good (file sharing is on, discovery is on, etc.) and no settings have been changed at all.
All of the other Windows 7 clients can connect just fine to these shared folders. We have also tried the simple things (restarting machines, router, etc.)