Lost XP product key, not the same as installed version

Hi,
I am sick of getting notified about the authenticity of my version of Windows XP and would dearly love to get all the updates needed. My version of XP is the absolute latest however I have doubts about it’s origins. Therefore I purchased XP Home Edition (2002) I held off reinstalling to this older but authentic version as long as I could. Now I wish to reinstal this version and have the disk but not the product key.

So, how can I reinstal my original authentic copy of XP Home over the probably non authentic XP Pro (2006)when I don’t have the XP Home Product key?

Thanks for any advice!