"...Is it true? .,.."

Very
If you dont have to login with a User name and Password then your installation is the "KNOPPIX" style.....



"...Is it possible to enable the login screen if this is the case?..."
Short answer is No.

Longer answer would involve editing /etc/rc*.d/ and removing the link for knoppix-xsession (something like that). And possibly a bunch of other ado. ... Best option is to reinstall using the Debian Option...... just make sure to save your Home Directory (/home/knoppix/) is the case that is contains data you have saved and ish to retain.



Take care,
-Justin
One Mynds Eye