Dear Erik,
Considering that you will probably want to access EPPI-Reviewer 4, then you wouldn't really have a choice. EPPI-Reviewer requires Silverlight. Why is it so? Silverlight is a Microsoft-plugin (for browsers) that allows us to have a web-page that looks, feels and behaves exactly as a desktop application would. With the added advantages that it works in the the same way (almost exactly) on both Windows and Mac machines and that we can guarantee that all users will receive the latest version whenever we publish and update. So, from our point of view, the advantages are unbeatable.

From the end user point of view, well, quite a few popular websites and services rely on Silverlight, but since you are asking, it seems that you never felt the need. On the other hand, Silverlight was designed with very strict security principles in mind, and as far as I know, no malicious activity has been documented to exploit the plugin (I know only of one phishing email that pretended to install Silverlight and installed a key-logger instead). In short, Silverlight is a small download, that poses very little (if any) security risk, so I don't see why one wouldn't want to install it (doesn't slow down your machine when not in use).
You can install it from here: http://www.microsoft.com/getsilverlight/Get-Started/Install/Default.aspx

Best wishes,

Sergio