This question has been asked in the forum many many times, and I think it deserves going into the wiki.
It may occur simply because some one has changed his password and forget to write it down, or you have purchased a second hand Eeepc, or you have changed the keyboard layout and so the Eeepc do not accept your old password.
Anyway, here is a summary of suggested solutions offered by different forum members:
This is the easiest way. Just press <F9> at boot up when you see the Asus splash screen, then select Restore Factory Settings from the boot menu. This will format sda2, the USER partition of your system, including all customisations and software updates you have done. On the 701 models, it will also wipe out your /home folder including My Documents. So not really a solution.
To do this, you need to delete a hidden file called .firstrundone in the folder /home/user/. To be exact, this folder is in the 2nd partition of the SSD (sda2) for the 701 models and in the 2nd SSD (sdb1) for the 900 and 1000H models.
So, if you can still boot up your Eeepc, e.g. if you have config “Log me in automatically when system start up”, then you just open File Manager, navigate to “My Home” (/home/user). From the menu, select View > Show hidden files. Select this file and delete it. Then next time when you boot up your Eeepc, it will run the first run wizard and show the license agreement, keyboard, user name, and ask you to enter your password, etc.
mount /dev/sda2 /mnt-user rm /mnt-user/home/user/.firstrundone
This will delete the file and after that, you should umount the drive, wait at least 5 sec and reboot pressing [Ctrl]-[Alt]-[Del].
If however you have a 900 or 1000H model, as there is no /dev/sdb1 in rescue mode, you will need to mknod to create /dev/sdb1 with this command before you can mount the /home folder (sdb1) to delete .firstrundone.
mknod -m 644 /dev/sdb1 b 8 17
Some background information first: In a standard Linux system, passwords are stored in the file /etc/passwd. If encoded, it will refer to another file /etc/shadow. Usually, simply deleting these files or edit them to make the root password field empty will do the trick. But in the Eeepc, the logon screen of the Simple desktop do not accept empty password. That is why this trick won't work in the Eeepc.
To change password, you can enter either of these commands in a terminal:
sudo passwd root sudo passwd user
[NB. the root user and the ordinary user share the same password in the Eeepc.]
If you cannot login your Eeepc now, then again you have to do it in rescue mode. The steps are as follows:
sh-3.1#passwd Enter new UNIX password: Retype new UNIX password: passwd: password updated successfully
I never use the Advanced Desktop, so I am not sure what is the difference logging in with kdm. So I will leave this open to other forum users to update.