P.Pilcher,
(a) If your orignal "scrapped" machine had an OEM license, then that license lives and dies with the machine. You cannot re-use it. Nor can you upgrade the motherboard in the same machine and re-use it.
(b) If you download from a questionable source, how can you be assured of the integrity of your copy ? How do you know there are no back doors, viruses, trojans or other nasties ?
(c) I believe (would need to double check) that Microsoft define a valid license as both the license key itself and the fact that you installed it using a valid bitset. I know this is certainly the case if you downgrade Vista Business ... you can re-use a license key but must use a valid bitset.