So I get five PCs with the very same hardware, I install all of them with Windows 7 Pro 64bits, and give each one a consecutive name (for example, PC1, PC2, PC3, PC4 and PC5) with group name GROUP.
We check they can connect to each other without problems, and that they connect to a test machine with XP Pro (by connect, I mean to see the network shares available) and here's the problem: when trying to see the XP shares, only one of them asks for a user and password in an imaginary domain named the same as the Windows 7 machine.
We have tried reinstalling the machine to no avail. It connects perfectly to Windows 7 machines, but it can't to the XP machines we have around.
UPDATE: It also fails to Windows 7 Home.