I just answered you in the other forum thread. For convenience I paste my reply below:
If it is the same setup as Arindam's and you are ajaxifying the
RadWindowManager for some reason, yes this could be the reason. The
good news is that this exact problem - RadWindowManager not finding its
windows in the Windows collection after being updated with Ajax - is
fixed in Q2 2009. For convenience I attached a small sample that shows
the problem that was fixed. If you run the sample with an older version
of the control, you will get empty collection after reloading the
manager with Ajax. If you are using Q2 2009 however, everything will
work as expected.
Please make sure that you are using the latest version of the control.
If you still experience problems after that, it will be best to open a
support ticket and to send me a small sample project where the problem
is isolated. I will check it and get back to you right away.
the Telerik team