One way to keep the window from opening is to hit the little graphic for the "last post", highlighted in this screen cap. Or if the topic is more than one page...if you hit the specific page number it will open in the same window (also highlighted in the screen cap).
If your searching by posts rather than topics...it will simply open the post in a new window/tab. I think most people will prefer this method after they get used to it, I can understand how it might be tedious using the "view new topics since last visit", but there are options to make it work the "old" way, it will just take a little bit of getting used to, but at least if you forget where to click to open in the same window...you haven't lost your place. Personally using a browser that uses tabs, I haven't noticed it at all, as I always opened opsts in a new tab anyway.