Thanks Shinu, for always trying to help.
But when I add the code snippet to my code I get error.
So I moved the radwindow declaration to the page instead of to Button2 click.
I get following error on Button1 click
Microsoft JScript runtime error: Unable to get value of the property 'show': object is null or undefined
I noticed that the menuwindow clientId was empty on the genearted script.
So I moved the radwindow from class to aspx page.
Now I don't get error but it opens my window plus an additional window.
Any help is much appreciated.