First of all, sorry for my bad English, as it isn't my primary language.

I have a home network with my Ubuntu box, a Windows XP box (sister's laptop) and a Windows 7 box (mother's PC). Never had a mixed network; with some effort I could install Samba and see the PCs (and be seen for them).

All Windows PCs can access Ubuntu, create and delete files and folders in the shared folders like a charm, and I can do the same with the Windows XP PC. From Ubuntu to Windows 7, however, I can't; a window requesting credentials appears, but there's nothing password-protected accounts in Windows 7. There's just one user account on that computer, without password.

How can I access this computer?