I had the same problem with my new Windows 7 laptop computer, which demanded a password, every time I tried to access shared files and folders on networked Win XP computers. However, it would not accept any password that I entered.
All of the network computers were in the same Workgroup - and I had the same User Identity on all computers. My User Identity is password protected on one of the Win XP computers. However, I have never set a login password on the other Win XP computer.
My home network also includes a NAS network drive and two network printers.
Although I tried everything suggested on this and other forums, the Windows 7 computer never allowed me to access shared files on the NAS drive or on the Windows XP computers. Furthermore, I could not connect to either of the network printers. There was
no obvious reason for the problem.
Then, I switched to a different ISP. My old ADSL modem and wireless router had been configured for the previous ISP, so I took the opportunity to upgrade my router from an old Huawei model (802.11b/g only) to a TP-Link router, which supports the 802.11n
wireless protocol.
To my surprise, the new router resolved the networking problems immediately - and the Windows 7 computer no longer asks for a network password.
I compared all of the settings on the old and new routers - and could not find any significant differences. The fact remains that a change of hardware has resolved the problems completely.