You can usually make the .iso discs from your own installer discs using something like UltraISO. I'm not familiar with not having the discs, to be honest. Anyone who has the need to dabble in multiple OSes would have access to those OSes already. Do you know anyone with a TechNet account, or that would let you borrow their XP disc? You only need the disc, but it has to be the proper version and platform for your existing key.