Put each installation into its own subfolder (i.e. c:/xampp/htdocs/wp1, c:/xampp/htdocs/wp2 etc.), access them via
http://localhost/wp1 and
http://localhost/wp2 etc.
Also apply different database for each installation (i.e. also WP1 and WP2 and WP3 or so, create these databases in PhpMyAdmin before installing WordPress).
Instead you might apply only one database for all installations, but in that case you have to apply different suffix/prefix (see WordPress installation dialog) for each installation.
For any deeper question go to a WordPress forum and ask for support, as this is NOT an Xampp issue, but a WordPress issue. This forum does not provide WordPress support.