With ASP.NET 2.0

I want to add a nice and sleek feature to my application where the user
typical clicks a button to open a smaller window to enable a search, lets
say for customers or products or whatever.

So far this is easy by adding a script tag and window.open with appropriate
settings.

The challenge comes when the user has searched and selected his item, and
clicks [Ok] in this popup form.
How do I return the values to the calling form, into the appropriate
textboxes?

If this cannot be done directly on the client side, I think this may be done
with a postback of the caller form, but then; how do I force a callback of
the first form (the caller form) when the secondary (the pop-up) closes?

(And yes, I know the search issue can be solved with hiding/showing panels
or views on an asp-form, but I would like to simulate a more windows like
behaviour, if possible)

Bjorn