Make sure your NAS is configured to require a user name and password. When you try to access the NAS volume, and the login pops up, make sure you select "save to key chain." Then it shouldn't ask again. If you still get prompted for a password, you may want to open your KeyChain and remove all entries that pertain to your NAS.
Also regarding the login script question earlier in the thread, if you use the following in an AppleScript and reference the AppleScript in the login items, a window will not pop up--much cleaner than the solution posted above:
delay 10
mount volume "afp://192.168.1.7/Public"
The delay is so your computer has a chance to connect to the network first. The IP address and subfolder will vary of course, depending on your NAS. If AFP doesn't work, you can replace it with SMB--it depends on what kind of NAS you have and what protocols it supports. Your NAS IP address should also be static.