watcher, ok, could you check what security properties you've got in Miranda NG folder when you put it in main directory of C: or D: ? That would explain difference between your and my test, because of list of users that have privilages to this folder. I've got only system user, owner, administrator group and user group - it should be also domain user group because my users were added to this group.
Also i could confirm that portable version works in some situations, because i tried this just seconds ago on different PC. But i'm still not completely sure about this problem. I need to check this.