> My Belkon router has aerials
OK, that means it is almopst certainly Wi-Fi.
> but I connect it with an ethernet cable.
That means you're not using the Wi-fi.
> I also have a spare wireless mouse which works OK althouh I prefer to use an ordinary cable connected mouse.
Nothing to do with Wi-fi whatsoever.
> However does that mean my desktop PC is WIFI enabled
No, it means your router is wireless enabled. Your PC probably isn't.
> and I would be able to use a Kindle ereader or any Laptop wirelessly.
Yes, because they will connect via Wi-fi to your router - not your desktop PC.
> Or do I need to find a WIFI hotspot in my area. ?
No, you don't.