Hi esm,
If you're seeing mumble-jumble code, this is an instant indicator that something is configured wrongly... Try RE-Installing XAMPP, which may be the best option, because then you can do a clean install of your OScommerce program/scripts... This is probably the best thing...
Give it a go... there's sufficient documentation on the main XAMPP website to show you how to install XAMPP properly....
Give it a try, just remember to backup any of your scripts that you made and configured... etc...
Try it.... If you're feeling confident! LOL
Good Luck...
God Bless,
Mitch