Got it working that way in sparky xfce. I have switched to xfce for old machines, it runs ok on an Pentium 4 with 512 MB RAM, has better translation to spanish and more consistent interface for users migrating from XP.
XFCE autologins after fresh install (selecting the option during setup) but after system upgrade autologin stops working, don´t know why.
DID this (on 2 pentium 4 machines):
1. open terminal
2. type "su" (without quotes) and hit enter - (this will made you temporarily be root user which is necessary to edit important system files)
3. it will ask for password and it won't show letters while typing, put password hit enter
4. type in without quotes: "mousepad /etc/lightdm/lightdm.conf" and hit enter - (this will open lightdm.conf file in text editing program called mousepad)
5. under [SeatDefaults] (be sure not to search under: # Seat defaults), find line: #autologin-user= (after quotes there will be some comment), delete whole line (with # and comment), and replace it with: autologin-user=yourusername (instead of "yourusername" put your real user name)
6. under [SeatDefaults] (be sure not to search under: # Seat defaults), find line: #autologin-user-timeout= (after quotes there will be some comment), delete whole line (with # and comment), and replace it with: autologin-user-timeout=0
7. click: file-save, exit mousepad, exit terminal
8. restart computer and watch it logging in debian without prompting you for username and password