Well if you are doing the processing on the webform, you could have a persisting string and do some javascript commands between them.

I'm not positive of the exact scripts you would need to write, but here is a sample script to get your going and show you how to dynamically place a client side script on your page.

Code:
strReportPagePopup = "<script language=""JavaScript"">alert('This is my popup');</script>"
Page.RegisterClientScriptBlock("msgBox", strReportPagePopup)
I hope this assists you