IT sounds to me like the issue is different versions of IE, or different browsers in general, because that is what renders your page, not XP or Windows 7. Every new web dev goes through this, you need to learn to use CSS that works in all browsers, and even then, you need to test in as many as you can.
IETester[
^] is awesome, install it on W7 and you can open your page in all versions of IE and see how they look.