It is not unusual to come upon used machines with a valid Windows Product Key sticker but without installation media. Although Windows XP is approaching end-of-life (April 8, 2014), it should be serviceable for awhile after that date.
http://www.zone54.com has a link to a “torrent” that will download “Windows XP Professional 32 bit SP3 Black Edition”. Microsoft would not enthusiastically endorse this site but with the COA sticker on the case, you could go so far as to put your product key in the computer (utility provided).
I tried installing the iso on an aging IBM Thinkpad R52 and it works flawlessly (almost). There will be a little driver cleanup to do but networking came up instantly. Service pack 3 is slipstreamed as are all the updates through February 17, 2013. It requires neither activation nor registration. Much better than any other Windows ISO. I did download an OEM copy of Windows XP Media Center Edition 2005 OEM but I think this iso makes that obsolete.
I am going to toss this guy a few bucks. I was about to spend $40 on Ebay for a copy of a Microsoft OEM disk, Bought the laptop for scottsbargains.com for $50, and a battery and charger off Ebay for $30. Microcenter had the memory upgrade to 2GB for $16. All-together about $100 for a functional Windows laptop.