Two quick 'n dirty JavaScript snippets to determine whether Flash/Silverlight are installed. Wrapped in try/catch blocks to prevent IE8 from throwing errors. Uses the "navigator.plugins" object in modern browsers and ActiveXObject for old IE.

detect.js

var hasFlash = false;
var hasSilverlight = false;

try {
  hasFlash = !!((typeof navigator.plugins != "undefined" && typeof navigator.plugins["Shockwave Flash"] == "object") || (window.ActiveXObject && (new ActiveXObject("ShockwaveFlash.ShockwaveFlash")) !== false));
} catch(err) {
  hasFlash = false;
}

try {
  hasSilverlight = !!((typeof navigator.plugins != "undefined" && typeof navigator.plugins["Silverlight Plug-In"] == "object") || (window.ActiveXObject && (new ActiveXObject("AgControl.AgControl")) !== false));
} catch(err) {
  hasSilverlight = false;
}