How can I install Ubuntu as the only OS on a virgin harddrive ?

Boot to the installeation CD, Check the CD for defects then continue the boot. You can then use the entire space for Ubuntu.

I personally suggest you use custom partitioning and make 3 partitions rather than Ubuntu's default 2

1) primary / (root) partition of 10Gb format to ext4
2) primary swap space made the size of your RAM (unless you have less than 2Gb RAM then use 2 x RAM) to swap
3) primary /home partition for the rest of the space formatted to etx4

This will not only enable EASY reinstalls (just mark the root partition for formatting), but also makes backups easier. It also keeps user data and system files seperate. If the drive is SSD and quite small then adjust, you can probably get away with 6Gb for root but if its a big platter based drive more than 80Gb then 10Gb will give you no worries at all.