ok, it looks like Wayne already answered it than ;)
here is more info
http://support.microsoft.com/kb/307859

and there is nothing wrong with XP, hey if it works, why upgrade :)

Thanks. One problem though; it's not opening a new program when I open the file, it's opening Firefox like if it were to display the page, and just displaying the HTML code in Firefox like it was a text document.

Again, do you have a webserver installed on your PC?

If you now have a web server running, you will need to put your site files into your "www" folder (the location of "www" will be different depending on which route you took to install your webserver). Then you will access your sites through localhost (or whatever your server name, but more than likely it'll be localhost). ie: http://localhost/site_folder_name