I was immediately suspicious when I got a MacKeeper pop-up and did not click or load anything. Called Apple immediately, but Tech Support is closed. At first was very concerned because I could not seem to "open" Safari--each time I hit my icon, I just got the pop-up instead. Was starting to panic cuz couldn't talk to anyone and couldn't get to this site cuz couldn't open Safari.
But then I simply closed the popup window (red x in top left corner) and Voila! Now I get my start page in Safari again. I also checked my system and could not find a trace of MacKeeper in my Applications file--so not sure how the pop-up itself got in.
However, am still waiting for callback from Apple Tech Support re how to prevent future pop-ups.
Don't know if this helps anyone, but I will inform after I talk to Apple tomorrow at 1:00 pm Pacific.
PS Noticed they had a similar "trojan horse" called Mac Defender back in March that they issued a software update for...maybe something similar for this?