At a guess, I woul say you need to access the Boot.ini file.
First from RUN type in MSCONFIG
then go to Boot.ini and run the 'Check all boot paths' button.
This should (hopefully) find that the NAS device is not connected and correct it, otherwise you will need to access the Boot.ini file directly.
To do this :
from search type in boot.ini. Activate all system and hidden files. When you find the file make a copy calling it boot.bak (in case of an error, so you can recover it). Open it in NOTEPAD, no other program is suitable.
Remove the line the specifies the non-excistant NAS drive, save and re-boot.
This should now default to the normal harddrive system. Terry