paying to download win7 from microsoft?

Hey,

so on my laptop i have win7 ultimate and my main pc i have vista home premium which i want to upgrade to 7 ultimate and on the microsoft website it says you can download it and they can send you a backup disk for no extra charge unless you want it in 2 business days then its an extra £20

but was i was wondering if i put download only what will happen? do i download a .iso and burn it to dvd? or can i install from usb? how does it work?

also if i download it form another pc say my laptop would it allow me to install it to my main pc or only from where i have downloaded the file?

thanks for the info .. i think i will press download only save it to usb will be much easier then having a cd.. save to a 64GB usb

thanks! that would be very handy espesially as you download the .iso from microsoft and i happen to have a spare 16Gb flash drive i can have as a windows cd sweet.