It is a long running bug in VirtualBox that if “Guest Additions” are installed in Windows Guest OS, after a while the OS seems to hand with extremely sluggish behaviour. The task-manager shows VBoxTray.exe as the culprit with 100% CPU usage.
Killing VBoxTray nukes guest additions functionality such as Clipboard transfers, drag-drop file transfers. In fact, restarting VboxTray does not restore the functionality. A Virtual Machine restart is required.
The discussion thread at https://www.virtualbox.org/ticket/15478 is yet to be solved.
An easy to demonstrate the problem would be to enable drag-drop and then simply drag a file and hover over the Guest OS. There is no need to drop the file. Simply hovering over the Guest OS with a file causes VBoxTray to spike CPU usage.
Disabling Drag-drop file transfers immediately restores the situation to normal.
Check-out the screen-recording for an illustration of the problem and solution.