if your copy of xp needs to be re-activated enter the activation screen and choose the option to activate without registering. if it wont activate online simply call the freephone number on the screen (only free from a landline phone) and follow the instructions on the phone. you will be directed to a call centre operator.
they will ask:
what changes you have made to your computer.
tell them you changed your hard drive
they will ask how many machines your copy of xp is installed on.
tell them only one.
they may ask where you got the installation disk
tell them you borrowed it from a friend but the keycode is from your machine's sticker
if asked tell them the xp sticker is on your machine
stick to your guns!!!
they will give you a code over the phone and talk you through the activation process
you now have a fully activated and legal copy of xp.
THEY DO NOT ASK FOR PERSONAL INFORMATION
I have done this many times and have never had a problem.