To run a Silverlight applicaiton on a client machine you don't need anything.
Once you open the application in the browser, the Silverlight plugin will install itself automatically (after a prompt of course).
If you are planning to setup a development machine, you need some basic components.
However, this depends on what version of Silverlight you are planning to use on your machine.
Check out
http://www.silverlight.net/getstarted/[
^].