I have in my stuff an old Compaq laptop. Recently I installed the HP Windows XP service pack 2 and it works normal. No activation was needed. The OEM CD is still for sale on eBay. Also available are the drivers the might be needed. These should cost less than $20.
I believe this works because HP bought Compaq sometime ago and the OEM Windows software apparently knows about this and does not need activation.
Anybody that wants XP on a old machine that does
not have an OEM CD available can buy a retail version of XPSP3 for about $30 and it will have the legal product key needed.
Some older machines might not have all drivers anywhere. You wail need to find them yourself. Of course, don't expect Microsoft to help you .
But there are people here that might help.
EDIT: Forgot to mention; Major Geeks has an 'SP4' for Windows XP.