Fixed site under the popup, but scroll inside it

Hi,
I have links between different div's inside my popup and that works great when I use "position:fixed;" in the css. The problem is that my popup is quite big so I would like to use "position:absolute;" so that you can scroll to see all of the popup window even though you have a smaller screen.
When I use "position:absolute;" and links to a div inside my popup the whole site is moving, not just the content in my popup. I hope you understand my description. Is there someone who knows a solution on this? Thank you!