I'm guessing that XP won't start because it doesn't recognise the drive you're booting from as the one it booted from the last time. While copying a partition from one drive to the other should theoretically work, windows does store some info about the drive it's installed on somewhere in its innards.
Suggestion:
Copy the partition to the new drive using Ghost, or something similar.
Get out your XP CD and boot from this.
Now run the setup and install (without formatting or changing anything about the partitions) in the exact same location as your current installation.
I must add that I haven't tried this myself yet, but this way you should end up with a working example of your trusty XP version again, including all your old settings, programs etc.