Q: Mac Mini, what Windows software can I install?

That's an old computer... but you could install Windows 7 on it.

That particular model can only install up to Windows Vista using bootcamp. Not Windows 7.

System requirements to install Windows on your Mac using Boot Camp - Apple Support

You can probably install later versions using something like Parallels or Virtualbox, but performance will be limited by the available RAM.

As to where to get copies of Windows, that would be a question for Microsoft.

You could probably buy them off Ebay, or the like though.