HTPC won’t recognise network password

I’d be grateful for any thoughts on this. I’m trying to set up a newly-acquired HTPC running XBMC 13.2 on a small Windows 7-based home network, where video files are stored on the main computer.

On the XBMC machine, if I try to add the main PC as a source I’m prompted for a username and password. The username seems to be recognised and is displayed in the dialogue box, but when I enter the password (asterisks appear as usual) and click Done the box reverts to being empty – is this supposed to happen? If I then click OK, what seems to be a list of machines on the network flashes up very briefly and then I’m returned to the username/password dialogue.

Clearly I’m doing something wrong, but what? I’ve checked both username and password and they seem to be correct, but is what’s happening a sign that I’ve entered something that’s not right?.

Thanks in advance for any help. I’ve looked in the Wiki of course but can’t see anything directly relevant.