Following is the procedure to resolve the SMB/CIFS authentication problem from a windows 7 machine(not able to access NAS from a windows 7 machine).
1) Start>run>secpol.msc
2) Local Policies>Security Options>
3) Change to –Network Security:Lan Manager authentication Level to send LM& NTLM-use NTLMv2 session security if negotiated
4) Start>run>regedit
5) HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Lsa
6) Change lmcompatibilitylevel to “3”
7) Reboot the computer
Then we can map the NAS drive from windows 7 machine.