It may be possible for you to bypass the Setup Assistant by creating a file at
/var/db/.AppleSetupDone. This can be done by: booting up into single user mode.
The following command will mount the boot drive as read/write device, create the
.AppleSetupDone file, and restart your computer
mount -uw / && touch /var/db/.AppleSetupDone && reboot
If your user account is actually intact, you will be able to login to the computer, and everything will be normal. However, in some of these situations, it's possible that your user account doesn't actually exist (unlikely based on the fact that you can't recreate it), or that your password hash is not a supported one (or is missing).
If you cannot login normally after disabling the Setup Assistant, you can re-enable it by removing the file that you created above. This can be accomplished by booting back into single user mode, and performing the following command:
mount -uw / && rm /var/db/.AppleSetupDone && reboot
Once you've restarted, create a differently named user account (Admin?), and use that account to survey the scene. With the newly created user, make sure that you see an existing account for your previous user in System Preferences. If the user exists, it may help to reset the password for that user (to re-create the hash), and then attempt to login.