I realise i posted this in help, but im desperate

This is what happened. I installed Windows XP on my computer, but since it wasnt a legit version, I did not activate it using the phone or the internet options.

Now that I have a new, legit Windows XP CD, I need you guys to help me get rid of the previous installs on my hard drive and instal this new CD. When I start the computer with the New CD in the CD tray, it doesnt load it, it just goes to asking if you would like to activate windows. So how am i supposed to install the new CD?

Or can I activate an un-legit copy of windows using a legit License key?

Thank you in advance