Comment The best solution is to lock down Silverlight (Score 2, Insightful) 153
For plugins like silverlight that run code rather poorly sandboxed, you should lock them to a whitelist, so that only web sites you have preapproved can use them.
Additionally, you should only run them on an unpriviledged user. (Something many Windows users don't do with anything as a regular practice.)
These two measures won't eliminate your risk, but they will dramatically reduce it.