1. Stop MySQL service. Run services.msc as administrator and stop it.
2. Run cmd.exe as administrator.
4. Run another cmd.exe as administrator.
6. Execute query:
UPDATE mysql.user SET Password=PASSWORD('newpasswd') WHERE USER='root';
7. Close cmd windows.
8. Kill mysqld process from processes list in task manager.
9. Start MySQL service. Run services.msc as administrator and start it.
10. Open phpmyadmin and login with your new password.