Windows xp reinstall question.

I recently installed windows xp professional onto a friends Dell Inspiron 6000. It originally had windows xp home. It's up and running like brand new, so there aren't issues. But from what I've been reading it's suppose to not work without some kind of registration key or number or something. And I didn't even know this at the time I installed it, but it did work and it wasn't hard to install. Just wondering what I'm missing here, this has to be common. The windows xp professional disc came from another Dell computer.

Thanks,
Jeff

 

It is because you used an SLP version of XP.

http://en.wikipedia.org/wiki/System_Locked_Preinstallation

 

I knew someone would know the answer!!

Thanks,
Jeff