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<iframe id="hotspot" src="starterpage.html"></iframe>
Then in the links that you want to have change the iframe's content, include a target="hotspot" attribute in the anchor tag.
Be warned that search engines will not index the entire window's content as one page -- because the iframe's content is really in a different html document. So search engines will probably send traffic directly to the page you intended to be shown inside the parent page. You should account for this, either with some scripting to put the orphaned page back in its iframe, or at least some friendly links that will let visitors load some configurations of the parent page (use target="_top") if they continue to explore the site.
There are more fine points and possible attributes to learn about iframes. Some good information here (as usual):
[w3schools.com...]
An online demo to play around with:
[w3schools.com...]
[frame noresize site:w3schools.com]
And because the term [nblue]site:w3schools.com[/blue] is included you will get only pages on w3schools that talk about frame and noresize. You can also use that kind of search for any domain name, including WebmasterWorld, to do a nice focused search on whatever topic you are investigating.