I've never had much luck trying to repair Windows installations, particularly from a different CD than that from which it was installed. Aside from that, I don't think anyone here can/will advocate downloading proprietary MS products.
Both repair installs and fresh installs are kind of a crap shoot in my experience. A big factor is where your friend has her important data saved. If it's in a C:\foo\ drive or somewhere like that, you can sometimes re-install windows (without reformatting, obviously -- I think the install-time option is something like "leave current filesystem in place") without FUBAR-ing everything, but that is usually a big hassle since it can mess up shortcuts, the registry, start menu, etc. If it does come down to having to do that, use a live-CD to copy as much important data as you can to an external HDD or other storage and you can possibly import it back into her bookkeeping, stock control, etc. software.
You can try calling up MS and telling them that you did a repair install and want to activate your windows. Tell them you're at the "do you want to activate" screen and are going to click the "yes" button now......"uh, operator, nothing seems to be happening...what do I do?" and see if they can give you some support there. They may just tell you to wipe the whole thing though.
I'd try this once more before wiping everything though. If it does come down to that unfortunate scenario, consider putting in a second HDD or partitioning the HDD such that the OS is on one HDD or a small partition and everything else (personal documents, business data, etc.) is on another HDD or partition; that way nuking the OS and reinstalling it isn't as big of a deal.