OS X: Spontaneous Application Appearance
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.