You can consider using an ATLAS UpdatePanel, updating only the content as needed. This is a fair amount of work, however, if you want it for links in the site.
You can also consider using some form of frame, but frames tend to be somewhat frowned upon.
You therefore need to consider whether the effort is worth it, or whether you want to jsut accept the normal default behaviour of the web, where clicking links takes you to a new page and thus the entire browser window refreshes.
Dave