Hm, your code works here and the window shows. Double check your encoding is definitely Unicode, and that your XML file is called "Windows.xml" (as referenced by your script).
If you haven't already, you could also download the Interface Tester program (linked to in the Scripting Docs under "Testing your Windows") and try opening the window in that. If the window opens then the XML is fine and the problem lies elsewhere.
If you've checked all of the above and it still doesn't work, post back here. I cant think of anything else that could cause it, but someone else may have some further ideas