subdomain mulitsite on localhost in a subfolder

Multisite cannot be installed in a subdirectory of the document root currently. This will be possible in 3.5.

See Multisite on Windows with wildcard subdomains for a guide how to manage subdomains automatically on Windows with XAMPP. You can move the plugin directory to another directory per wp-config.php. This is possible for themes too, but automatic theme upgrades will fail currently.