Maybe I paniced too quickly. Just found this explanation. Makes sense. I do have a Samsung phone and did plug it to a USB port to charge.
"I had this, also. There was no way I was going to click on either button, not knowing what it was. By ending processes one-by-one through the Task Manager until the the dialog went away, I tracked it down to a program called UA.exe. This a Verizon application to support Samsung Android phones. So, it should be safe to click on the 'Install Now' button. However, the developers of the software should be fired for being so stupid as to put up such an anonymous dialog box."
Edit: Sure enough, Samsung Upgrade Assistant dialog box came up. However, I tend to agree with the last sentence above.