Partitioning in the installer is quite straightforward, providing you know a few
things up front, and choose the right option. If you have one drive and you want
to replace an OS you choose use the entire disk. If you have two drives and you
want to devote one to Linux you could also use this option, providing that you
choose the correct disk. You could also choose Guided and it will step you
through it giving a bit more control.
I always choose Manual. This gives you full control.
Drives in Linux are not given conventional Windows names. They are either sda,
sdb, sdc, or hda, hdb, hdc, depending on whether they are SDA or IDE. Partitions
use a similar naming scheme, sda1, sda2, or hda1, hda2, etc. When using
partitioning you need to choose the correct drive. It is a good idea to pay
attention to the file system type and the amount of space used. NTFS indicates a
Windows file system and ext indicates Linux.
In partitioning for Linux you want to right click on the partition you want to
use, either on the ribbon graph or the table. Choose Edit from the drop down. Or
you can click to highlight and choose edit below the graph. When you edit you
want to do two things, choose the file system type and the mount point. It can
get more complicated as a good system will have more than one mount point.
The file system you want should be either ext3 or Reiser. The mount point will
be / which stands for root. You should tell it to format by clicking that box.
There are more advanced setups, but with a 20 GB partition you should keep it
simple. If you have more space you could divide into three partitions, / (root),
/home and swap. This can wait until you decide to give more space and turf
Windows ;).
Once you choose the partitions you can proceed to the next step. On page 7
carefully review the changes before they are committed on page 8. There is no
going back at page 8. On page six you will need to choose a user name and
password. Make sure you remember each as you can't logon without them. You can
control where the boot loader (grub) is written to by clicking the advanced tab
on page 7. Grub will usually be written to the bootloader of the drive on which
Ubuntu is installed. If you have only one drive this is a no brainer, but it can
become complicated if you have more than one drive. Writing to the wrong drive
will mean that Ubuntu fails to load and you will have to edit grub, post
installation. Let's assume that it goes well.
Finally, you have Vista and it can be problematic with Linux. Vista updates have
been known to pooch your dual boot by reclaiming the boot manager. Cross this
bridge if and when you come to it. There is lots on the net about dual booting
Vista and Linux or Ubuntu. Just do a Google search for dual boot Ubuntu Vista.