Reinstall El Capitan on 2009 iMac I've giving my 2009 iMac to a friend. It was running El Capitan and Windows 10 on Bootcamp. I've tried to be clever (which apparently I'm not) and I've done a clean reset of Windows 10, not understanding that I would no longer be able to see or access Mac. Yes, all traces of macOS have disappeared.  Even the Mac keyboard doesn't work, presumably because I've lost all Bootcamp drivers.  

The Mac partition may still be there somewhere, as the Windows partition only takes 130GB of the 500GB HDD. So somehow, I need to restore macOS using a Windows mouse and keyboard. Oh dear.....

Any advice would be much appreciated.
Find your original installation (grey) DVD or acquire a DVD that is not bound to the machine (e.g. "Snow Leopard" DVD with the animal picture, not a grey CD).
Boot from the DVD
Using the disk utility (available from the DVD), fully erase the drive.
Install the original OS (e.g. "Snow Leopard 10.6.3")
If you installed snow Snow Leopard, get the "10.6.8" combo upgrade package and install it.
Search on Apple's site how to install OS X El Capitan on old MacBooks. I remember the link is not easy to find, but there's one. Download it and install it.