What I don't know how to do, is how to re-install the WinXP bootloader on that drive.
[...]
Boot from installation CD, open "Recovery Console", and do:
bootcfg /rebuild
fixmbr
fixboot
I am late to this, and you got your answer. I can confirm that it works and it used to be the standrad way to install the bootloader. I suppose you may want to consider a way to do this w/o booting from CD if you want to keep using XP on newer machines with no optical drive. If so, you may consider BartPE by nu2(free version, and way more complete, of WinPE) to keep in one USB bootable medium the tools needed to make your HDD bootable and, as I used to back then, drive images and re-image tools.
http://www.nu2.nu/pebuilder/http://www.nu2.nu/ Once you have a botable .iso of BartPE, I suppose you can easily use current Linux tools to port it to bootable USB. Should you want to do this from your Windows XP box for some reason, here's an how it used to be done:
http://users.telenet.be/Robvdb/BootPEUSB_UK.htm