IANAL, but...
As other's have mentioned, what you own is the cd key. I believe you are free to use another person's windows xp cd, but you will have to make sure it's the same flavor(IE, if you own an XP home oem key, you will need a home oem disc, if it's pro retail, you will need a pro retail disc). So, if you still have your cd key, just ask around, and see if you can't get your hands on a physical cd. Once you get it installed, you may be able to legally make a copy of it, but I'm not 100% sure about that. XP discs have no copy protection, just search around for a slipstreaming tutorial to help you get the proper options in your burning software of choice.