The following command can be used to set the password for the root user and grant all privileges:
UPDATE mysql.user SET Password=PASSWORD('NewPasswordHere') WHERE User='root';
GRANT ALL PRIVILEGES ON *.* TO 'root'@'localhost';
FLUSH PRIVILEGES;
Place this SQL code into a text file and save it some place convenient. Then, kill the SQL server. Open a command prompt and cd to the directory where the mysql-nt executable is stored. Execute this command:
mysqld-nt --init-file=sql-file-saved-above.txt
Kill the SQL process you just started and restart the server normally and all should be good. Depending on how you have phpMyAdmin configured, you might need to adjust its configuration if you have the root password saved there.