You have options.
You can load the Live CD and go to Settings | Administration | Partition Editor.
Make sure your drives are not mounted first. Choose your drive in the top right
corner. Then you can do any number of things. The most radical would be to start
over by destroying all existing partitions, creating a new drives and partitions
and then reinstalling. Or you can destroy only some partitions and then resize
the ones you want to keep to fill the existing free space. This can be harder to
do, depending on how your drive is set up. Also if you go this route you will
likely have to edit grub to reflect the loss of partitions.
If you opt for the former then I would go with three partitions. this will give
you one for / (root), one for /home (user files) and one for swap. The format
for / and /home can be either ext3 or Reiser and swap can be left as swap. Your
swap drive should be comparatively small, not exceeding your RAM, otherwise you
are wasting space. You can decide how big / and /home should be based on your
habits. If you install lots of programs then / will have to be larger than you
typical 4CBs that most Linux distros need. If you download lots then /home will
have to be larger. If you are unsure split the difference.
Once done then you can install Ubuntu. Make sure that you choose custom or
manual partitioning. When the partition table shows, it will show the suggested
partitions with / meaning root and /home meaning home. A check in a box means
that the partition will be formatted. To change the set up, right click on the
drive name and choose to edit. In the dialog box that appears set your mount
point to one of the partitions as / and the other one as /home. Choose the
format of both as either ext3 or Reiser. Once this is done proceed and look
carefully at the changes before they are committed. If it is right then proceed,
if not go back.
Once you set up your computer with three partitions and have done it once,
re-installing is a breeze because whatever you do to one partition does not
affect the others. If you always do a custom installation you can repeat this,
but make sure that your home partition is used by setting the mount point, but
never formatting it. This way you can re-use your home folder indefinitely and
never lose any data or settings, even if you change distributions.
If you elect for the less drastic approach and destroy only some partitions and
resize the ones to keep, you may have to edit grub. If you keep the first
partitions and destroy the higher numbered ones then you may get away with not
having to edit grub.
If you do have to edit grub, it isn't that hard to do. Just change to root in a
terminal with sudo su or sudo -i and then launch gedit by typing gedit in the
terminal from which you will have to open the menu.lst file in the root folder
/boot/grub/. Since you will be working from the LiveCD, if grub is not working,
remember to change drives first be going to File System on the left and
navigating to the media folder, changing to your hard drive (the live CD works
from a RAM drive).
Once in grub you will have to change some numbers. If it says, (hd0,6) for your
boot partition then you will have to change it to (hd0,0) if your root partiton
is your first one (it could used sda instead of hda, depending on what kind of
drives you have). Keep in mind that grub uses ordinal numbers (starting with
zero for the first one) and numbers are relative. The first number is the drive
and the second number is the partition. For example, if you swap partition was
first, /home partition second, then your boot drive would be (hd0,2) because
your swap would be (hd0,0) and your /home would be (hd0,1).
If it was me and I had my preference, I would elect for the fresh start,
provided I backed up all data first. That way I could set things up properly and
not have to worry about grub editing. However, if I was essentially happy with
my old setup, I might want to opt for the less drastic approach.