Hi:
I can't help you with the 157mb disk problem.
Also, trying to use a recovery disk or image from another model notebook will almost certainly not work on your HP 620
All I can suggest is you try using the method I posted to make your own W7 plain installation media.
It has yet to fail on any notebook or desktop I have used it on.
The phone number I posted is the HP business PC support number for the United Kingdom.
Your HP 620 is a business notebook and obtaining recovery disks for business PC's requires a different process than getting them for consumer PC's.
That has always been the number where folks that live in the UK call to get recovery disks for their HP business notebooks and desktops.
You will have to call them on Monday and find out if you can still get recovery disks sent to you directly from HP.
They used to ship them for free in the UK--even if the PC was no longer under warranty.