You cannot do this unless you have /home on a separate partition. If you have
only one partition and everything is on that partition then you cannot save your
data. A new installation will wipe out your data. This is why it is advisable to
take the time to do a custom installation and create two partitions. One for /
and one for /home. Once this is done you will never have to worry about
re-installing and losing data again.

The situation is retrievable, if you want to go to the trouble of doing it. You
can use the Live CD to resize and create a new partition and then move your home
folder to that new partition. Then you can re-install Ubuntu to your old
partition and keep the data intact on the new partition. You can then just
change the mount points at the time of installation to reflect your changes.
Ubuntu will go on the old drive and the mount point will be / and home will go
on the new one and the mount point will be /home. You will want to format /, but
not /home.

If your installation is messed up you have nothing to lose. Just make back ups
in case.

Advice on this can be found here: http://www.psychocats.net/ubuntu/separatehome