Someone just sent me this tip as I was having the same problem after my web hosts initial install did an auto upgrade that overwrote the web.config file and messed up the default passwords to the site.  I didn't try this tip, my web host had reset everything somehow, but this might work for you:

"As there are frequent requests from users, who lost the password of the host account (and you cannot request the host password by email), the following solution will help for all DNN 3 and 4 installation. Please note, that your need direct access to the database to suceed with the issue:

• if "register" is not displayed for the portal, go to table "Portals"  in your database and enter value "2" into column "Registration".
• create a new user account by registration (this time, please remember the password you enter!)
• go to database, enter table "ASPNet_Membership"
• go to new user account (usually the last one) and copy the encrypted values of columns "Password" and "PasswordSalt" into the same columns of user account "host" (usually th first entry in this table) 
• login as user "host" using the new password and delete the reshly created other user.
• Keep remembering your password ;) "

Hope that helps,

Rick