or basically, you need to remember the document.body.scrollTop in a hidden input control, see the following:
Reset the page scroll position after a PostBack
http://www.codeproject.com/aspnet/AspNetScrollbarMove.asp
Is it possible to prevent a Web form from scrolling to the top of the page when it posts back to the server?
http://www.wintellect.com/resources/faqs/default.aspx?faq_id=1&page=7#6