Clean installation can be done from the live CD
without formatting ! The easiest way: Boot up the live CD, then give the command
sudo nautilus in the Terminal and access the hard disk. From there, delete all
the folders except for the Home folder. Enter the Home folder, click on View,
Show hidden files and delete everything that begins with a dot ., like .emerald
, .evolution and all the other system files that begin with a dot.
 Then, install Ubuntu with the last option of the installer, manual
partitioning or something like that. There do not check the option for
formatting your partition and such you will do a clean install with preserving
all your data.