Have a look here how to reset your password: http://www.howtoforge.com/forums/showthread.php?t=13652
Error.. I see it's not the admin password you are talking about.
You Encrypting RSA private key, and now everytime you restart your server you need to enter the password.
In step 7 ("Encrypting RSA private key of CA with a pass phrase for security [ca.key]")and step 8 ("Encrypting RSA private key of SERVER with a pass phrase for security [server.key]") of the certificate creation process you are asked if you want to encrypt the respective key now. Choose n there because otherwise you will always be asked for a password whenever you want to restart the ISPConfig system which means it cannot be restarted without human interaction!
Have a look here how to fix it: http://www.howtoforge.com/forums/showthread.php?t=14073