I installed Mandriva and Ubuntu on the 20 GB unpartitioned space very easily.
I had to reinstall both. Mandriva has a very efficient and easy to use
"partition table". I booted from the Mandriva CD, deleted the Mandriva and
Ubuntu partitions, created a partition for Mandriva and let the rest
unpartitioned, then installed Mandriva on this partition. Then rebooted, booted
from the Ubuntu CD and tried to install Ubuntu on the remaining free space, but
did not show this option for the first time, so I quitted and tried for the
second time and succeeded. At the second time it showed the option for using the
available free space.