My laptop came with Window 8, which has a radically different interface
You could always install Classic Shell, an aftermarket launcher for Windows, to put the S back in Window 8.1 and give you an interface that's closer to Windows 7. Android likewise has aftermarket launchers.
of course I pulled out the HDD, installed an SSD and put Linux on it
Which is like installing a custom ROM on an Android device: there's ABSOLUTELY NO WARRANTY that all peripherals will be supported. I still haven't got my laptop's Bluetooth working in Xubuntu.
Oh, and there's 32-bit and 64-bit
And ARM vs. MIPS vs. Atom.
and Home and Pro and Basic and Ultimate and...
That's more a matter of which OS component repositories you're allowed to access than actual OS fragmentation.