Can the modal window still disappear if the user clicks elsewhere? Or do they need to click somewhere inside in order to continue? If that's the case: don't.
I think I have an understanding what you are trying to do here. Basically you want the user to see search results, but you don't want them to navigate to a separate page, right? Do you want them to be able to view all the search results, or just the most relevant set?
Either way, I think an overlay can be the answer here. As the user puts in keywords in the search bar, fire of a search and let a overlay appear. In the overlay, you'll get some results. You can either choose to just display the most relevant ones, or go more extreme and offer all search results (you'll be needing pagination for this though.).
If you want an example, visit the dutch website Tweakers. You can see a search bar just below the navigation in the top. Type in some keywords (iPad review for example) and have a look :)