Installing Linux Ubuntu

Hi All:
This is somewhat OT but I hope someone can help.
My son is learning Linux at school and wants to download Ubuntu onto
his laptop, which doesn't have a CD drive.
Is it possible to download from the Ubuntu site onto a USB memory stick
instead of a CD?
If the drive on the laptop is then re-formatted to load up Ubuntu, can
the laptop be made to boot from the memory stick so the install can
happen properly, or does wiping the drive of the existing operating
system make it unable to load from the USB port.
The machine already has a version of Linux on it...does it even need to
be wiped to install Ubuntu or will it just overwrite the OS that's
already running like Windoze does?
All help gratefully accepted on behalf of my rotten kid!!!
Cheers

Marcus