If you want to keep using this current install ...
Boot from that live dvd, choose the "try ubuntu" option, click on the disk in the launcher and try to find your system (it will have a /home/ directory) and click /home. Your username is the directory name in there (could be more than one but seeing the names should be enough I hope).
Then all you need to do is reset the password for this user. Regarding resetting the password I would refer you to How do I reset a lost administrative password?
Regarding the other parts in your question:
I could just update to a new version of Ubuntu if there is a new version and if it would delete the user accounts (doubtful).
No, you can not unless you retrieve username and reset the password. Updating will need you to log into your acccount ;)
I am very willing to delete everything and start again.
That is also an option; a re-install will remove anything related to Ubuntu and have you start from zero. With a new username and a new password. But try not to forget at least the user name. Be smart and pick something easy as a name (your own name is always a good option).
Mind though that if you do have personal documents it might be wise to put them on an USB before re-installing (also possible from the "try ubuntu" option).