I have a 9.04 (Jaunty) ISO burned to a USB stick; it appears to be a "live DVD". When I boot from it, I get a GRUB menu listing:

• Ubuntu, with Linux 2.6.35-generic (This matches the system currently installed on the HDD?)
• Ubuntu, with Linux 2.6.35-generic (recovery mode)
• Memory test
• Ubuntu 9.04, kernel 2.6.28-11-generic (on /dev/sda1)
• Ubuntu 9.04, kernel 2.6.28-11-generic (recovery mode) (on /dev/sda1)
• Ubuntu 9.04, memtest86+ (on /dev/sda1)

When I select Ubuntu 9.04, kernel 2.6.28-11-generic (on /dev/sda1), I arrive at the desktop of a 9.04 system. I want to wipe the HDD clean and install 9.04. (Upgrading to something newer is not an option; this version is required by a legacy application.)

How can I install from this live USB image?

I vaguely remember some incantation that I should be able to use in the booted system, but my google-fu is broken at the moment.

I'm comfortable with low-level commands, so if you want to recommend a more hard-core strategy, I'm willing to roll with it without requiring a ton of detail...