Just set the first setting to something "\\.\PhysicalDrive1"
Then point it at a folder with an empty 'xp' virtual machine. Do not overwrite the one that is there and create a new one. If you point it at the existing one the thing will not even boot due to the way virtualbox 'tags' virtual harddrives with a guid.
Plan on it taking a couple of hours to clone.
Then detach the old drive and attach the new one under the IDE controller. https://www.virtualbox.org/manual/ch03.html#settings-storage There will be a little hd with a plus over it (the pic in the docs is the old gui). That is add hard disk button. Just use "chose existing disk" button and go from there.
Make sure you match the memory size and number of cpus you had in the old box.
Also make a snapshot right after you get it all setup and before you turn it on. That way if you screw something up it does not mess up your cloned HD.
Fire it up and hope it works It probably will want you to re-register windows as it will see it as a hardware change.