Never ever run with admin rights on an account you use everyday. Anything nasty comes near you and its curtains.
A very fair point and normally I would agree with you (and certainly would if others too used the machine) but to cut a long story short I ended up making a convenient decision to run as an administrator. In the relatively unlikely (not impossible, granted) chance that it gets infected it will quite easily be rebuilt from a combination of clone (of the partitioned HDD with os and apps and nothing else) and backup (of data to the other three HDDs). I will only blame myself if I need to do this!
To the problem; as mentioned before my money is on the PC as all other machines on the network do not have this issue when connecting to the NAS. I am pretty sure the NAS is not asking for a username and password.
Will do some more digging ce soir.