OS X: Spontaneous Application Appearance A friend of mine asked me to inspect her computer a while ago as she suspected that she may have some virus trouble.
In part of my initial check of her computer (MacBook Pro OS X, probably Mavericks) I found 3 applications in the Applications folder, which according to her: "spontaneously appeared". 
These applications included Microsoft Silverlight, MacKeeper and some other arbitrary media player, and after a small amount of research, I found that they may in fact be malicious software (adware, spyware or malware) so I promptly uninstalled them:
Problem solved.

My question is: 


Can Applications just "Appear" in the Applications folder without User interaction/authentication?


She insists that she did not install them or give any authentication surrounding them that she remembers, which is extremely unusual as I would not expect programs to have that degree of access without user interaction.

Does anyone have any ideas?
Silverlight is non-malicious, it'a a Microsoft 'plugin' similar to Flash [at consumer-level]. It will have been offered as the user hit a web page needing it &amp; would require admin acceptance before it would install.

MacKeeper is not so much malicious as insidious. It's is an 'all in one fixit' app with dubious marketing strategy.
It often 'features' on any page vaguely relevant to Mac care &amp; maintenance, with the idea that it is the most obvious button to click, instead of the one you meant to.
In that respect it is a 'PUP' [potentially unwanted program] although no-one appears to have actually credited it with being intentionally malign - opinions on it are loud &amp; varied.  

Neither of these could have just sprung into the machine of their own accord, they would need user complicity, even if it was unintentional/misleading.

You'd have to let us know what the arbitrary media player is, to comment further.