Hi Devilsan, I understand what you want to do but I don't know how to do it. But what I do instead is keep the latest update files on a usb key. Then put the usb key in the non-networked pc, run avg update & choose folder update rather than internet update.
The update files for the AVG free edition are here for download:
(set of .bin files)
slightly different url than for the paid edition, not sure if it makes any difference.
Not a lot of help in relation to your precise question I know but hope it might be of some use.
I'd like to know how to do it the way you're asking, too, if anyone has that info.
Awhile back when I was hunting for the above link, I did find a few newsgroup posts via google that mentioned the location of the update files that avg normally downloads itself. As I recall the anatomy of the free edition is markedly different from the paid edition. I never noted the info down because the download-from-website-onto-usb-key method seemed orders of magnitude easier, but the info is 'out there' somewhere