windows network password

i have a small home network - two pcs running windows 7 ultimate, a seagate black armor nas and a sony blu ray player. everyone was talking to each other without issue. my first pc was getting a bit full/tired/slow so i decided to rebuild it with an ssd boot drive. new fresh install of windows, everything went fine until i tried to connect to the nas - "enter network username and password", there is no user name or password!!! i eventually, through trial and error, managed to gain access with "username - admin, password - admin". however, two problems remain. despite checking the 'remember credentials' box it doesn't. whenever i reboot i have to enter them again, not a massive deal but a little annoying. big problem however is that this still only gives windows access, not other programs - so the seagate nas discovery software cannot access the nas - "error listing shares of device: access is denied" and serviio, which i use to push the media stored on the nas downstairs to the blu ray cannot access it either. most annoying. the weird thing is that i only built the second pc a few months ago, exactly the same set up, but this one can access the nas without any issues. i have been trawling the forums and have found loads of people with the same or similar problems, but no solutions - i have tried all the solutions offered with no success (plus the other pc is able to access the network without changing any settings). any clues?