how safe is 'microsoft silverlight'?

With SilverLight, Microsoft attempted to create their own version of what a replacement for Flash could be - but not to universal acclaim. You can read more about it here:

http://silverlight.net/

So, if you go to any sites that have been designed for this new Silverlight stuff, you can download the plug-in from here. Version 2 only works with Intel Macs and does work with Safari 4/5. The alternative solution is to download and install the latest version of Flip4Mac, which now includes a Silverlight component:

http://dynamic.telestream.net/downloads/download-flip4macwmv.htm

But the latest news is that Microsoft will probably abandon Silverlight in favour of HTML5:

http://www.appleinsider.com/articles/10/10/29/apples_ios_pushes_microsoft_to_dia l_down_silverlight_for_html5.html and

http://www.bbc.co.uk/news/technology-11673384

If you want to remove Silverlight the instructions are here:

http://www.microsoft.com/getsilverlight/get-started/install/removing-silverlight -mac.aspx

Recommendation: Close all applications before installing any of the above, repair permissions, reboot, and repair permissions again.