Thanks Shayley, pleased you're engaged as I've spotted lots of +ve comments about your feedback in other threads.
O.K. Right now in IE9 I'm getting It works! for http://localhost/index.html and Server error for http://localhost/index.php
I may have made things worse since my original post, I'm desperate to solve this so I've been trying all sorts of things to rectify it. Yesterday I was able to open up the XAMPP Admin page in IE9 and Firefox. As far as Firefox is concerned, I tried loads of suggestions I found on this site and others and eventually decided to uninstall / re-install Firefox 8, no success, and then uninstall Firefox 8 and install Firefox 4, no success. As IE9 is my default browser, Firefox now doesn't display anything, it merely prompts IE9 to open the file with the same result as above.
Today, I'll be trying to retrace my steps so that I'm back where I was yesterday as if you'd asked me the same Q yersterday I would have said 'It works' and the XAMPP Admin page (at least in IE9).