Here is the link to XP licensing, it may take a little effort to read but is fairly straight forward:http://www.microsoft.com/windowsxp/eula/home.mspx
13. SOFTWARE TRANSFER. Internal. You may move the Software to a different Workstation Computer. After the transfer, you must completely remove the Software from the former Workstation Computer. Transfer to Third Party. The initial user of the Software may make a one-time permanent transfer of this EULA and Software to another end user, provided the initial user retains no copies of the Software. This transfer must include all of the Software (including all component parts, the media and printed materials, any upgrades, this EULA, and, if applicable, the Certificate of Authenticity). The transfer may not be an indirect transfer, such as a consignment. Prior to the transfer, the end user receiving the Software must agree to all the EULA terms.
As you can see, the XP license was more flexible that Win 7 OEM.
That said, you likely will never get it working right now without the installation media. If you don't have it, you don't own the license. If you don't own the license, this post violates our forum rules.http://www.tomshardware.com/forum/245458-45-read-first