IBM Thinkpad HDD swap without boot disc?

I have two IBM thinkpad 600e notebooks, one is in good condition, has a 6Gb HDD running windows 98 the other is being used for spares and has a 3Gb HDD running XP PRO sp1 (this literally fills the HDD). I would like to wipe the 6GB HDD and install XP from the other HDD. I figure I can put the 6Gb HDD into a caddy and format it on any PC. The problem is I cant find my xp boot disc or product key number and dont want to buy a new one or use an illegal copy. How do I copy the 3Gb HDD contents to put it onto the other HDD so it will work in the good 6 Gb nootebook straight away?