Creating new localhost site without completely losing access to another

I’ve got an older (2009) installation of XAMPP Lite on my Windows 7 PC that I use to sandbox a website using localhost. It also has older versions of WP (3.6) and Thesis (1.85) that I’m reluctant to update for fear it will break some of my site’s features that will need to be fixed manually. I don’t want to put the effort into fully updating this local site since I’m the only one who sees it, but I would like to keep it for reference.
I now want to create a second local website with up-to-date versions of WP and Thesis and am wondering how to go about it. It would be nice to start with a fresh install of XAMPP, but from what I’m reading, it would conflict with my existing XAMMP Lite installation. But I can’t completely uninstall the latter because its directory contains all the docs for my existing site.
Then there’s the issue of accessing two local sites. The instructions for setting up a localhost multisite WP network look daunting enough to make me think I’m better off sticking with a single localhost site. I don’t mind setting up my new site up that way, if maybe there’s a way to manually change some settings to point it at my old site on the rare occasions that I need to access it.
I would welcome any advice anyone may have for setting up a new localhost site without losing all access to my old one!