This is strange behavior as Apache in XAMPP will always write the error 500 details to the error.log file if the server is running.
Do you have any more servers running on your PC.
Does Apache and MySQL appear as Running
in green in the XAMPP Control Panel?
Check by running xampp-portcheck.exe found in the C:\xampp folder on a default installation and make sure that port 80, 443 and 3306 are occupied by XAMPP's server components.
in your browser and see if the orange XAMPP Welcome Demo page is loaded - which should also load if you just type http://localhost
If not what page is loaded?
You can try this by looking for a .htaccess file in the C:\xampp\htdocs\joomla folder and temporarily move it out of the folder and try again to load http://localhost/joomla
- usually a 500 error is caused by an incorrect entry in a .htaccess file.
BTW how are you trying to set the root user password - details please.
Using the security page linked in the Welcome Demo Page is preferred and is documented in the readme_en.txt file.