0) You may have to use the original hard drive as usually you can't install and run Windows off of an external drive (Assuming it's USB or Firewire, I have no idea about eSATA drives).
1) They should. I can't think of any reason it wouldn't work on a Laptop if it works fine on a Desktop.
I have only done on a XP/Vista dualboot on a desktop though so I can't be 100% on that.
If you are installing XP 2nd you will need to fix the boot loader so both will work using a guide like this one.
2) You can look on HP's driver page at http://welcome.hp.com/country/.....ay=drivers to see if there are XP Drivers available for your model laptop.
Edit: I just found the thread http://www.howtogeek.com/forum.....-hp-laptop.
It appears you may have problems even if you install XP on your original hard drive.
That thread has instructions on making a custom XP disk with the hard drive drivers on if needed.