This issue has been done to death when 1.66 and other old versions used a deprecated WinMySQLAdmin and the advice then and now is not to use it but to use instead phpMyAdmin.
You can safely delete the 3 winmysqladmin files from \xampp\mysql\bin\ folder which will then make the Admin... button redundant in the XAMPP Control Panel - just in case you are tempted to click on it again by mistake.
Remove from the Windows Startup folder any reference to winmysqladmin.
You can also kill the process if you find it in Windows Task Manager.
Here are some posts found by using keyword winmysqladmin
in an advanced forum search in the XAMPP for Windows
There were many more to read if you have the time.
To set the root user password for MySQL and for accessing phpMyAdmin (both use the same root user and password) click on the Security menu item found when you type http://localhost/xampp
in your browser.
Why not install a newer version of XAMPP, for instance 1.7.3, which seems to be stable and does not include winmysqladmin but links to phpmyadmin from the MySQL Admin... button in the XAMPP Control Panel.