Download Mavericks Free?

Question

I run Snow Leopard. Twice a pop-up message appeared on my MacBookPro saying "Download Mavericks Free."
This box is highly suspicious. It has a URL called "Details" and nothing else. Just those 4 words, and no way to delete the box.
We know what clicking on a URL from an unknown source can lead to, and no one with sense would do it. But there is no x to delete the invading box. No way to get rid of it except reboot.
No reputable company would do that, certainly not Apple, so I'm wondering who sends it, and if it can be blocked. Of course my browser is set to block pop-up windows, but hackers have no problem skirting that.
Anyone else get this attack?

Answer 1

The announcement is from Apple. Mavericks is free and they constant reminders are annoying after a while. Yosemite due out this fall will also be free.
OS X Mavericks: System Requirements
To install Mavericks, you need one of these Macs:
          ▪          iMac (Mid-2007 or later)
          ▪          MacBook (13-inch Aluminum, Late 2008), (13-inch, Early 2009 or later)
          ▪          MacBook Pro (13-inch, Mid-2009 or later),
          ▪          MacBook Pro (15-inch or 17-inch, Mid/Late 2007 or later)
          ▪          MacBook Air (Late 2008 or later)
          ▪          Mac mini (Early 2009 or later)
          ▪          Mac Pro (Early 2008 or later)
          ▪          Xserve (Early 2009)
Your Mac also needs:
  ▪ OS X Mountain Lion, Lion, or Snow Leopard v10.6.8 already installed
  ▪ 2 GB or more of memory  <--I would recommend a minimum of 4GB but 8GB is much better.
  ▪ 8 GB or more of available space
While your computer might meet the requirements, also check your applications for Mavericks compatibility here:  http://roaringapps.com/