There is a 2nd (D drive showing on the explorer tab rather than within the PC tab, any ideas as nothing has changed and this just showed up after starting his system.
Go into Start menu and type in 'Disk Manager' and click on the option that states "Create and format hard disk partitions" (or Disk Management). Open that and then post a screenshot of that instead. I would suspect you've just got a partition if you've not installed any new drives.