It will install on a new machine as easily as this one. OEM and retail disks are the same, just the packaging and licensing is different. (as said earlier, so long as the disk is not machine specific image disk, but from an online retailer it wont be, just be carefull of
ebay).
At worst, it may refuse to activate online because it's already been activated already. I
think the retail version may be more tolerant of this. However, as Phil says there has to be a way around this, incase of major hardware failure. If the activation fails, you get a UK phone number and a code on screen, you phone the automated service and follow the instructions, and in fact there is an option to say the motherboard has been replaced, it then gives you a reply code to type in and activates XP.
It used to go through to an operator, but is fully automated the last few times I did it. However, if it's longer than about 6 months (IME) it just activates with no need of the phone call - I assume they must clear the logs.
there is nothing dodgy in it IMO, you build the PC so are entitled to OEM software. MS have recognized this issue for home builders and I believe they have relaxed the rules a bit for Win 7.
The rules are there to stop shops just reusing the same licence all the time.