Since you are using XAMPP I assume you installed phpMyAdmin yourself. You should have had to setup the username and password during setup/install time. I believe there is a file config.inc.php that should contain the username and password for the mysql connection phpMyAdmin should use.
Check this link out
phpMyAdmin 3.3.0-dev - Documentation
If you do not know this you will need to figure out the root username/password MySQL was installed with. You can then get access.
phpMyAdmin is a tool installed for CPanel users. There are however some very nice GUI tools put out by mySQL themselves that you might find useful.
MySQL :: MySQL GUI Tools