I have a 80 GB hard-disk also After some trials in which I messed up my
computer I made the installation in the following way: I booted from the Windows
CD, wiped out the partitions, then made a 20 GB C partition, a 40 GB D partition
and I left the rest unpartitioned. Then I installed Ubuntu on the unpartitioned
hard-disk by choosing the option "Use largest available free space" This is the
best method, I think. Do not touch the red cursor on top because you might mess
up your Windows ( I did twice this mistake).
If you have 3 Windows partitions just copy your files from the third partition
somewhere else, delete the third partition by using a Windows CD, quit Windows
setup, reboot, boot from the Ubuntu CD and then install Ubuntu on the "largest
available free space".