MSN's site will fille the browser window if you screen resolution is 800x600. Any higher and the site has gaps on the sides. I noticed that when I went from 800x600 to 1024x768. The same is probably true for other sites as well.
Some sites, like MSN, most likely use tables to put their whole pages into and tables you can make any size you want. Other sites don't use tables so they will end up filling the window, regardless of your screen resolution. The only thing you can really do about it is change your screen resolution, but there's nothing wrong with your computer. As long as you can see everything on the page, that's all that matters.
I hope this clears that up for you.
You still never explained what you mean by "jams some lines together".