A suspicious dialogue box is appearing on my Mac, with a checkbox for using searchmoose. How can I get rid of this?

This morning when I booted up my MacBook Pro, I got a suspicious-looking dialogue box that says "Update Alert!" and something about downloading a utility in preparation for "the new version of OSX." A checkbox below offered searchmoose as my search engine. I unchecked that and clicked on Cancel, but the dialogue box appeared again a little while ago. It looks almost like an official Mac message - but not quite. Any suggestions about getting rid of this greatly appreciated!