Transferring Windows 7 License To A Totally New Machine

I have a very close friend who is having pc issues, is short on cash, and needs her pc for work. Where she lives (Russia) PCs are insanely expensive and there really are not any decent ones available. I offered to build her a new PC from scratch to cut down on cost, but the cost of a new windows install is prohibitive. She already has a valid windows 7 home premium license with her old PC, and the old PC will be going in the trash after proper cleanup, or possibly being reused with ubantu or something. Either way, the old PC will not be running windows 7 anymore. I figured I could install windows 7 fresh on the new PC and use her old license key to activate it once she gets it. Is there any issue with my logic here?

Here's where things get complicated. There is obviously a lot of distance involved, so I will have to assemble the PC and then mail it to her. She is not PC savvy, so having her do really any of the installation work is out of the question. She needs her old PC to continue functioning, poorly as it may be, right up to the point the new one gets there. Mail takes 3 weeks or so, which means it is absolutely out of the question that I activate the new pc before mailing it, since I assume that would knock the old one offline and she can't be out of work for three weeks, or even 3 days for that matter.

I need the new PC to be as close to ready to go for her as possible on arrival. I would love to be able to install all of the updates, antivirus, core programs she uses for work, etc before mailing it off so that she can just boot it up, call Microsoft and get it authorized, but I am not sure if this is possible. I know you can use a thumb drive with a downloaded official copy of windows 7 for the install, which I already have, so that part is not the real issue. The issue is whether I can load everything up for her prior to sending it off, because if I can't it's a no go for her. She'd rather take credit and pay for an overpriced hunk of junk that wrestle with a windows install. Not rational, I know, but such is life and it is not going to change.

So what I'm really asking, is whether I could use an official copy of windows 7 home premium on a thumb drive, and get the system the whole way up to operational before I register it? If need be I have a disc copy as well. Usually it says you have so many days to register your product, but will it allow me to install the updates etc? I don't want her to have to sit around for a day doing updates and installing programs.