You say that the popup window is opened with this code:
win = window.open(url.asp)
Understand then that the variable "win" returned from the window.open function is an object reference to the "window' object of the newly opened popup window. However, the snippet of code you gave above doesn't tell the whole story because of this commonly performed mistake:
If the variable "win" is declared within the function then there's no way you can reference the returned object outside of that function. The "win" variable should be a global object or attached as a property of a global object so that it can be used later. If your "win" object isn't declared global then it's possible that's why you can't get the suggestions provided here to work.
win = window.open(newURL);