There's a link to page where you can download the 'official' older versions on each of the 'official' download pages :
The highlighted link takes you here ==>>
Previous calibre releases
Sounds to me like you might have inherited some restrictions imposed by the original owners of the laptops to prevent installation of msi's - i.e. corporate lock down. I've been away from XP for too long to suggest anything other than install a fresh copy Windows XP, preferably from MS media.
If you can shoehorn another gig of ram into the laptops then maybe you could run Win7 or Win10 on the Intel Dual Core CPU's.
BR