Error: 'Access Denied For User

I have recently upgraded to mysql-4.0.23 from mysql-3.23 on linux system.
I started the server using the command: /usr/bin/mysqld_safe --user=root &
When I try to shutdown the server using the command: mysqladmin --user=root shutdown
I am getting the error as follows :
/usr/bin/mysqladmin: connect to server at 'localhost' failed
error: 'Access denied for user: 'root@localhost' (Using password: NO)'

And hence to shudown the server,i always need to kill the running mysqld process.I dont think its advisable to do this always.

Please ,help me in this regard and tell me the right command to shutdown the server.