Well I whish that it boiled down to a permission issue for everybody! In my case it has not.
unlink() fails permanently under php/windows with a 'permission denied' error; this is despite all privileges being adequate. It seems that it is caused by the readonly flag, which is permanently set on Windows XP folders, where it means something completely different (a problem which the various posts on the net, including Microsoft's KB, do not help to resolve). My best guess is that the php unlink() function verifies this flag and does not even attemp the operation.
So, when all else had failed, I had to design a workaround. Fortunately, it works fine: