From the recovery console you can reset any password, including the password of any users in the sudo group: http://www.psychocats.net/ubuntu/resetpassword
If you don't have the password, the best way to go is the recovery mode. If there is an account with sudo privileges (aka an admin account) of which they didn't give you the password, reset it using the link in this post. If you can't even find such an account, create one using the links in my previous post.
And indeed, a guest account doesn't let you do anything permanent.