Mysterious download/install

This morning I found an unexpected install dmg on my desktop and the little window asking for my password. 'Get info' produced: /private/tmp/tmpinstallmc.dmg A quick web search indicated that it was something called 'Genieo' and that I would not want it on my computer. I clicked 'cancel' on the window asking for my password and moved the icon to Trash. A quick search with Spotlight and I found the Genieo in my applications folder and some files in the Library folder - all binned. I emptied trash and restarted the iMac.

I have 3 questions:

1. Is there anything else I should do - anything I have missed?

2. How did that thing get onto my desktop in the first place ( I have not downloaded anything new for quite a while)?

3. How do I prevent it coming back (assuming that I have got rid of it altogether)?