useradd: cannot lock shadow password file

I got the following error, when I tried to add a new user
using the adduser command

useradd: cannot lock shadow password file

When I searched the forums, and I was able to recover from
this error by following the steps given in the following link,

http://www.justlinux.com/forum/showthread.php?t=128244



Well, the lock file will (most likely) be called:

/etc/shadow.lock

Now, in theory, it was probably created when you were using it at one point, your system crashed, and the lock file was never cleaned up. (Things like this used to happen for Netscape all the time.) However, it is quite possible that another program is using the shadow file. You may want to check to see if there's another process that is locking the shadow file. Run 'ps auxw | grep shadow' to see if anything is using it.If not, it would *probably* be safe just to delete the shadow.lock file. 

Remove /etc/shadow.lock file