Thanks Dwarf, for your suggstions, but I already have these: SSID is not broadcasted, WPA2 encryption and MAC filtering.
My guess is that the problem is rather that the DNS-320 itself in some way uses Linux or UNIX, which is not compatible with Windows 7, but works fine with Windows XP SP 3 (the other computers in my WLAN).
If I create a folder on my Windows 7 desktop, the security tab in "Properties" look like this (it is is Swedish, but you will probably understand it):
If I copy the folder to one of the volumes on my DNS-320, the same tab looks like this:
As you can see, the group and user list is changed and none of those listed has any access. I cannot change anything; either access is denied or the list returns to previous when I click OK.
So what causes this and how can I work around iy?