in phpMyAdmin, there should be a User Tab
listed in the users section is the root / localhost user, make sure that user is set to have al privileges.
this thread al stack overflow has some details on how to reset the root users privileges:
http://stackoverflow.com/questions/1709078/how-can-i-restore-the-mysql-root-users-full-privileges
if all else fails, export your databases using the export tab in phpMyAdmin, uninstall xampp and do a fresh download install of it. then re import the databases.