questo l'ho preso da quirksmode con qualche modifica per IE

codice:
<html>
<head>


</head>
<body>

<SCRIPT LANGUAGE="Javascript">
<!--

var flashinstalled = 0;
var flashversion = 0;
MSDetect = "false";
if (navigator.plugins && navigator.plugins.length)
{
	x = navigator.plugins["Shockwave Flash"];
	if (x)
	{
		flashinstalled = 2;
		if (x.description)
		{
			y = x.description;
			flashversion = y.charAt(y.indexOf('.')-1);
		}
	}
	else
		flashinstalled = 1;
	if (navigator.plugins["Shockwave Flash 2.0"])
	{
		flashinstalled = 2;
		flashversion = 2;
	}
}
else if (navigator.mimeTypes && navigator.mimeTypes.length)
{
	x = navigator.mimeTypes['application/x-shockwave-flash'];
	if (x && x.enabledPlugin)
		flashinstalled = 2;
	else
		flashinstalled = 1;
}
else
	MSDetect = "true";

// -->
</SCRIPT>

<SCRIPT LANGUAGE="VBScript">

on error resume next

If MSDetect = "true" Then
	For i = 2 to 100
		If Not(IsObject(CreateObject("ShockwaveFlash.ShockwaveFlash." & i))) Then

		Else
			flashinstalled = 2
			flashversion = i
		End If
	Next
End If

If flashinstalled = 0 Then
	flashinstalled = 1
End If

</SCRIPT>



<script language="javascript" type="text/javascript">
<!--
if (flashinstalled == 2)
{
	if (flashversion)
		document.write("You have Flash version " + flashversion + " installed.");
	else
		document.write("You have Flash installed, but I can't detect the version.");
}
else if (flashinstalled == 1) document.write("You don't have Flash installed.");
else document.write("I can't find out if you have Flash installed.");
// -->
</script>
</body>
</html>