A. IE 8 running on Windows XP and IE 8 running on Windows 7 share a lot of rendering code for HTML, CSS, and others, it's important to understand that there's a very different security model in place in IE on XP than on Windows 7. Just because something runs fine on IE 8 under XP doesn't necessarily mean it will run fine on IE 8 under Windows 7—especially web applications that do more than just show basic information. IE 7 and later on Windows Vista and above has the Protected Mode feature that isn't available under XP and could affect many aspects of content functionality, including web-based applications and browser extensions.
My recommendation is to test on IE 8 under Windows 7. If your site passes, it's likely the site would also work on IE 8 on XP (a less secure platform). To be 100 percent sure, you should primarily test on IE8 on Windows 7 and repeat some test cases on IE8 on XP.