My screen looks good in IE8 under Windows XP. But when I open it in IE8 under Windows Server 2003, all text is more bold than in XP. I can apply styles to it, for example change font-weight in css, but in that case font weight changes under Windows XP too and in any case my screen looks differ.
This is likely to be related to the available fonts on the computer rather than the browser.
Check what font the text is trying to use, and what fonts are installed on both machines. If there's any missing one one machine you could install them, or just set the stylesheet to use a font that is available on both machines.
If you're only worried about those particular machines running IE8 (ie if it's a site on your own internal intranet) then that should be enough.
However if you're expecting the page to be viewed externally, bear in mind that an exact pixel-perfect match is no possible across all browsers and operating systems. For example, if a Mac user opens your page, they certainly won't have Ariel and Tahoma fonts installed. The browser will try to pick the closest match it has, but it will look different. There's no real way around that. Even changing the screen resolution will affect how your fonts look.