I can connect to the NAS file server fine from a windows XP.
I have made sure the windows 7 and the NAS box have the same workgroup.
But the windows 7 machine cannot access the file server.
Answer:
Control Panel - Administrative Tools - Local Security Policy
Local Policies - Security Options
Network security: LAN Manager authentication level
Send LM & NTLM responses
Minimum session security for NTLM SSP
Disable Require 128-bit encryption
Also make sure you do not have any stored credentials for the NAS server. If you have, remove them.
Control Panel -> Credential manager
Do a restart of window 7.