Can't help with the 'My Book World Edition 11. White Light' as no experience of them.
Regarding your personal data files, don't do backups as such, just do straight copies of the files.
one way to do it is just take the HDD out of the pc that is to be scrapped (I'll call it SlaveHDD) and hook it up to your other XP computer (so that it is a slave, secondary drive) and then copy your personal Data files (My Documents) to a new folder on your SlaveHDD. So that you end up with all your personal files from both pc's on the SlaveHDD. Then disconnect the SlaveHDD.
You can now wipe the XP comp that you are keeping and install W7.
Now you can hook up the SlaveHDD again and copy back your files to the W7 install.
And same for your new Computer when you get it.
Others may have different suggestions, I use a usb connector when I do this sort of thing.
This is the type of lead that I use USB to SATA & IDE Drive Adapter click here mine has a power adapter with it.