Please note if your have a SP1 key but no CD. Microsoft can supply you with a new CD (there is some minimal cost)
This is for users with mising or damaged discs
Now because XP SP1 is not supported it will be a bit gray. ie Your request will be on the missing part more than the revision (ie you could have had XP SP1 and updated it freely to SP2 then SP3 and then the disc went missing - so therefore your issue is with XP SP3)
It really depends on who you speak to at MS and if they want to be helpful
Or just track down someone with a SP1 Setup CD