Windows 7 - Windows 7 Upgrade via download

Aplogies if this has been asked before but I couldnt find any previous thread on this. I have a student who has taken advantage of the Microsoft offer and ordered via download Windows 7. This turned out to perform an upgrade of Windows 7 over the existing Windows XP Home Edition. I have been tasked with upgrading the hard drive on this laptop over Christmas so would like to know where the install files will be for me to rebuild the OS once the new hard drive has been installed. I have asked them to check for an ISO file but they tell me there isnt one. Does Windows 7 just download and install wiping off any traces of the install files, if so, whats the procedure for re-installing in the event of hardware failure?

Any help grately appreciated

I think you might want to read this thread here http://www.sevenforums.com/installation-setup/32758-requesting-iso-digital-river.html

and here is the one to make a ISO out of the down load
Make bootable iso from student d/l