Here is a step by step guide:
....softpedia.com/.../...untu-8-04-LTS-84314.shtml

IMPORTANT: Be sure to back up your home partition to a DVD so that if it does
mess up you can just drag the files from the DVD to you new home.

You should choose to do a manual installation. When you do this you have to tell
Ubuntu where to put things. You want to replace your old Ubuntu with the new one
and format the partition where it will be placed. To do this you have to set the
mount point. This is not your home partition, just your Ubuntu root partition.

Manual partitioning will load the partition manager. You will have a table of
your drives and partitions. Select the drive first. It will then scan the drive
for partitions. It will display the available partitions and will choose one of
them. By right-clicking on the partition you can change its properties. For the
Ubuntu installation you will want to set the format to either ext3 or Reiser and
make sure that there is a check to format it. The mount point should be set to
/.

The home partition should not be checked to format. This is very important. Set
the mount point to /home.

Before Ubuntu begins installing it will show you the proposed changes. Read them
and make sure they are what you want. If they are not what you wanted, go back
to the previous screen and do it again.

When it asks you for a user name, you may either use an existing user name in
which case your previous data and settings will be used or you may choose a new
user name and manually bring over the data and settings after the installation
is complete. It is important that you NOT choose to import any settings from
Windows or other partitions as these will spoil the effect of having the same
set up as before.