So, even if the VM is on or off, you can't move files using drag & drop, correct? If that is the case, this is not a VirtualBox issue, you have to look somewhere else for support on this. This is a fault in your Host system, VB has not nothing to do with that. If it happened after you installed VB, try to look in your Add/Remove programs list to see what else installed on that day, and the days before. One of them might be the culprit.