A. what error are you gettting?
B. Are you using phpMyAdmin? - if so whom are you logging in as?
Explanation on XAMPP & MySQL :
XAMPP doesn't set the security on MYSQL, you do. All MySQL fresh installs have 'root' set to no password (null). The only way XAMPP affects the MYSQL configuration is if you have run the 'Security script' found here => http://localhost/security/xamppsecurity.php
<= It was created to make it easy to get MySQL & phpMyAdmin working.
If that script was run the 'root' password and 'pma' users passwords were set by whomever ran that script.
You can run 'resetroot.bat' to set the 'root' user back to no password, but you will have to edit config.inc.php manually.