You may have to adjust the level of encryption used by your windows 7 machine in order to authenticate to a device such as the NAS.
First try this,
open the network and sharing center and choose Change advanced sharing settings from the left column, scroll down to the section that says "File sharing connections" and choose "Enable file sharing for devices that use 40- or 56-bit encryption. Save changes and reboot machine. See if that helps.
If not post back, with the version of Windows 7 you are using and we will try another technique that may resolve the issue. It's different if you are using Home Premium as opposed to Pro, Enterprise or Ultimate.