use norton ghost it is the most famous one, just put your larger disk as slave, run the xp as usual, and you use the norton ghost to copy entire hdd to your new (slave) hdd.
after that restart your pc and go to bios so it will boot on your larger hdd. after going into xp, you can format or do freely with your old hdd...

of course if you just one time using this software instead of buying ghost, you can go to acronis.com, they have a fully working 15 days trial.

thanks i will give that a go