Tablets like the Kindle fire do not show up as USB external storage, the way e-ink ereaders do. They use the Media Transfer Protocol, which is not available for Windows XP.
According to that, your Kindle Fire should never have worked with your netbook, although the Sony would work.
Your options are:
- Buy a new computer, with a modern operating system.
- Install a modern operating system on your current computer. This means linux, since Windows 7 likely won't run. I suggest Linux Mint.
- Give up, and only use your Kindle Fire with the calibre Content Server.