problems about shared folder from samba(linux file serv

Hi, all

I have a file server on linux box. It's used to work fine on Win xp. Now I have new Win 7 box on LAN. It seems works when I map a drive on one shared folder from file server. It didn't ask me user name password and automatically connect. While on XP at the first time connection XP will prompt for user name and password. And I have an old application, which use the file server, runs fine on XP. Now on Win7, it can't access remote drive by complaining no permission nobody. I think Win7 use nobody as user name to connect the remote drive. Is that possible I can change the remote drive login user name and password. I mean let Win7 prompt for a user name and password when connecting to remote drive.

I think my problem is similar: I connect to a NAS (Western Digital Mybookworld, where I opened a ssh channel, so I can control linux box there - with basic linux and samba) from Windows 7.
Usually I choose its name (\\mybookworld) in explorer and it connects without asking any password.
After 4-5 minutes of inactivity Windows can't access anymore the linux box and seems to me an authentication problem, since other pcs in the network continue to communicate with my nas. If some program was connected, it crashes and it's impossible to kill the process from task manager.
Then I try to put IP address of my NAS it reconnects for some time, then the same.
I must reboot everytime, but due to hanging programs windows does not reboot. I have always to switch power off brutally.
I tried to remove credentials, to activate/deactivate ntlmv2 and ntlm and lm authentication via registry (windows 7 home premium has no gui for that, you must pay some fee more to have a fully functional system?)
No way
I tried to shut down every firewall
Nothing

I don't want to reinstall Windows XP. Anybody there can hel us?