Screen resolution differences. If the site template dictates that the page is 1280 pixels wide and your monitor is set to 1024 then you will have to scroll while someone using a monitor set to the 1280 size or greater will not have to scroll. An alternative reason may be any reading aids such as larger text standards like magnifier in Vista or its equivalent in windows XP. Try using ctrl & + (plus symbol) and CTRL & - (minus symbol) to adjust the font size manually.
On a final note what is the sites URL (address) and just as a side comment you should of placed this in the webdesign forum as you would get a better reply in general there.
Can anyone please help me to cancel Magnifier in Vista. I am recently new to Vista and I inadvertently clicked on "Magnifier" - up it came, and does so every time I boot up - but I cannot get rid of it! Can anyone brighter than me suggest a solution to this annoying thing!!!