Problem installing 7.10

I'm a complete newbie to Linux and Ubuntu. Am trying to install from the CD. Have previously cleaned and defragged my Windows before installing Ubuntu, and have an 80GB hd with over 70GB of free space. I've got stuck at stage 4 of 7 on the installation CD. At stage 4 I get the following options:- "Guided - use entire disk" (which I do not want to do), "Guided - use the largest continuous free space", and "Manual". Of those three options, I chose "Guided - use the largest continuous free space". When I select that option, I get a dialog box saying "Failed to partition the selected disk - this probably happened because the selected disk or free space is too small to be automatically partitioned". When I click OK, I get back to the box headed "Prepare disk space" (stage 4 of 7). This time, additional options are shown, as follows:- "Guided - resize IDE 1 master" and "partition #1 (hda1) and use freed space". I selected "Guided - resize IDE1 master", but then got a dialog box saying "No root file system - no root file system is defined. Please correct this from the partitioning menu". After clicking OK I get a box headed "Prepare partitions" (still shown as step 4 of 7). The options are as below:-

/dev/hda
  /dev/hda ntfs /media/hda1 82335MB unknown (if I highlight this and click OK I get a dialog box saying "No root
  free space 8MB file system is defined")

There are 2 buttons at the bottom, "New partition table" and "Undo changes to partitions".

I don't know where to go from here, and obviously want to avoid wiping my installation of Windows.

Any advice would be gratefully received!

Simon