Well...the key phrase was "nothing is left of the original system (hardware)."
If it were an OEM license, issued by either MS direct (such as I have several of) or one obtained via a boxmaker (Gateway, Dell, eMachines, Compaq, etc.)...that license died with the original equipment.
But...I would still try to activate it
. My thinking is always to let persons charged with making authoritative decisions...make those decisions, based on their evaluations of facts/premises presented. I have changed various equipment items on my systems and never had a problem...but I've not tried activation on a system totally lacking original components.
If it were not an OEM license, it still might be a point of contention, since MS rewrote the rules (a couple of years ago, I think) in terms of transferability of licenses. It did not apply to me, so I did read it...but it's at their website.
Bottom line regarding activation: You don't know until you try.
My own unsubstantiated personal opinion...is that the authorized personnel are likely to validate XP activation requests that are put forth with any legal, registered license key...while the bogus keys will still be denied. That's how I would be doing it, at this date, if I were in their jobs. But, that's just a personal, unsubstantiated perspective.
As for slipstreaming, very easy if you use a tool like nLite - Deployment Tool for the bootable Unattended Windows installation - Guide - http://www.nliteos.com/guide/
Downloadable SP2 is available at Download Windows XP Service Pack 2 XP SP2 (Full package) - http://www.softwarepatch.com/windows/xpsp2.html