KDE requires the /tmp directory to have the correct permissions (1777) for a user to log in or it will give you an error saying: “call to Inusertemp failed. temporary Directories Full?”. For this you have to change the folder permissions of /tmp by:
chmod 1777 /tmp
This will give permissions of drwxrwxrwt. One problem I was having with BackTrack was that after every reboot, the permission would get set back to dwrwxr_xr_x. I realized this was happening due to BackTrack using the aufs file system to mount root. The aufs or AnotherUnionFS is used on the live CD to make it appear as writeable but in reality the media is physically read only.
To eliminate this problem, open fstab:
And replace the aufs line with your device and the correct file system.
For example: /dev/sda8 / ext3 defaults 0 0