dbs - Are you asking about different browsers (IE, Firefox, etc.) or about screen resolutions? After clicking over to your site, I'm guessing the problem is with screen resolutions. With my resolution set at 1024x768, which is pretty common, I do have to scroll horizontally to see the entire page, and horizontal scrolling is probably what you've had complaints about.
The simplest way to keep horizontal scrolling at a minimum is to set your own screen resolution on the lowest resolution most of your visitors are likely to use, while you're designing your site. That way, as you're setting up your pages you'll be seeing the pages as those visitors will. If you have CSB5, you have the capability to set table width as a percentage of the screen width instead of a specific pixel number, so that may be helpful.
-- What screen resolutions you have to design for will depend on who your site visitors are. "Techy" computer sites, for example, can usually afford to design for higher screen resolutions because "techy" computer people tend to use higher screen resolutions. If your visitors are less computer-centric, they'll probably tend to use lower screen resolutions. If you're getting complaints from regular visitors, that's a good sign that you need to design for lower resolutions. If your webhost gives you access to your site statistics, you may be able to find out what percentage of your visitors are using which screen resolutions.
If my guess is wrong, and you are asking about different browsers, you may need to describe the problem more sppecifically - There are differences in how different browsers will "see" the same web page, but they don't usually involve resizing.