please read this : How to get scrollbar position with Javascript?
Method 1 and then consider to store a cookie with the scrollbar position.
Method 2 or store the value server side in a visitors table on base of the IP
There is one more funny way.
Method 3
use anchors like <a name="foo">
and send your post action to the URL containing the anchor like this <form action="site.php#foo" ...
Its not so exact like the cookie version but it works somehow :))