GNU/Linux Desktop Survival Guide
by Graham Williams
Partitioning Options 
On first attempt when the partition option was presented it claimed ``This computer has no operating system on it.'' (seems to be referring to /dev/sdb1) and offers to ``Install side by side, choosing between them each startup'' (presumably noticing Vista on /dev/sda). It mentions that with the ``Use the entire disk'' option ``This will delete Windows Vista (loader) and install Ubuntu 9.04.'' This then ends up installing Ubuntu in a 2.5GB partition on /dev/sdb5!
So reinstall, and choose to ``Specify partitions manually (advanced)'', setting up the desired structure.