Most new hard drives come with software that will do this, usually on a
bootable CD.
If not, try the manufacture's support site for downloadable tools, usually
as an ISO image, which you then burn to CD.
Alternatively, try the ultimate boot CD, a free download from:
http://ubcd.sourceforge.net/
This CD image includes some of these under a category called "hard disk
installation tools".
Finally, there are third-party tools that can easily do this, such as Norton
GHOST or Acronis TrueImage.
By the way, all of the above tools will copy 100% of the old hard drive to
the new one, not just the system partition. They will also copy the boot
record, or make an equivalent new one.
Before booting into XP, be sure to disconnect the old hard drive and connect
the new one onto the same cable at the same spot.
As for the old hard drive, if you wish to use it, simply reformat it, or
better, delete all partitions and make new ones. These operations are best
done by placing the old hard drive in a USB enclosure, then attaching it to
your PC. However they can also be done while the old hard drive is still
attached internally, but below the XP level. That is, use some disk tools
on bootable CD, or on the ultimate boot CD under "hard drive diaghnostic
tools". Or, boot the PC directly from an XP CD and use the recovery console
to delete paritions.
CAUTION: If using a bootable CD to erase the old hard drive, it is wise to
first unplug the new hard drive, to avoid a potentially painful mistake.