I am not sure about how are you calling the OpenWindow function. If you are calling using OnClientClick of ASP.NET button, then you need to cancel the postback in order to show the window as shown below.
The UpdatePanel does not parse scripts and this is well known and related to the MS AJAX Framework and not directly to RadControls - you can find more details about this as long as a workaround in teh blogpost we prepared for this and which is available below:
You do not need to use so complex approach to show the RadWindow from the server but better use the simpler manner which is setting VisibleOnPageLoad=true as you have done. However, I assume that this does not work in your case because of some incorrect code., e.g I am not sure how you reference the RadWindow by its name - RadWindow2 - this is not correct and you should receive an error. Please, try the following syntax: