Key xp

Your key isn't necessarily on your CD or its case. Sometimes there's a sticker under the keyboard or in the front of the system manual, or in/on the sidepanel of the PC. For service pack 2 versions like yours, perhaps you kept the fancy certificate of XPness (with key) someplace you remember?

Alternately, of course you can install another OS like Solaris or MacOS X (you get to throw down for that, and it tends to need certain CPUs) or license a spare copy of WinXP, or pick another OS from guides and sources like OSNews.

Microsoft Windows XP has been declared illegal in its bundling by courts, and rootkit viruses shipped on Sony music CDs in the recent past, so do not go calling keys or sites illegal or piratic unless you must. You will want to run antivirus or ICE if you relax your trust criteria for software; if you know Microsoft XP release and update team people well, maybe you trust that better. Could we tell if they (or Symantec) sold us out to Zimbabwe; perhaps by tracking the trail of XBox points?

We cannot give out the key, as it will be illegal. What you can do is temporarily using you friend's key, or any other that work on the version you are installing. After installing change the activation key with your one, either with activation or with windows keychanger.
You may need to do activation by phone, and chose to speak to a representative in phone options.
There are also illegal keys on the internet (most popular is one starting with fckgw ...) but some of the pirate sites distributing them also spread viruses, and their use is illegal.
Moreover, with a non genuine Windows key your system will not enable some feature as automatic updates.

See
Change the Serial in Windows XP