The general behavior of a modal RadWindow is to be centered on the page and thus ignore the Left and Top properties if they are present. This behavior is by design as generally modal dialogs popup in the center of the screen to make sure they will take the user's attention.
The way to move such a RadWindow is by calling moveTo() after it is shown, i.e. either in the OnClientShow event, or immediately after calling radopen, for example:
oWnd = radopen(
or with the OnClientShow handler:
All the best,
the Telerik team