I need to make some assumptions regarding your questions in order to provide answers.
If the NAS device supports for NFS and CIFS access, when you open a file from a UNIX system, the NAS device will also allow the file to be accessed by a Windows system running CIFS assuming the access controls are properly configured. The underlying implementation of the NAS device has some bearing on the accesses. If it's implemented on top of Windows, the access may not be allowed based on the permissions that were set for the file (this is called a hard lock). If the underlying implementation of the NAS device is Unix (or Unix-like), the access will be allowed even though permissions indicate otherwise but an advisory message is issued (this is called advisory-level locking).
Another scenario is, if your file serving is being done on a UNIX system where you have the file open and you want to access the file from a Windows client through CIFS, that UNIX system needs to have SAMBA installed to mimic the CIFS protocol.
Not all NAS devices support both CIFS and NFS accesses so you must check with the vendor. There are add-in software programs (SAMBA, Windows for networks, etc.) that you can use if you are doing file serving from a standard server.
Evaluator Group, Inc.
This was first published in July 2001