Best way to buy windows?

[Golaboots]

Hi

I'm thinking of treating my old PC to a SSD drive and a windows upgrade from Vista which is currently uses.
Many programs seem to be for 7 or 8 only now and not available for Vista.

What's the best way to do this. Windows 8 is £90 or so at Amazon. That's not mega money but it is when the PC is probably only worth £120 or so.

I've seen a few places offering cheaper alternatives, is there a way to check these are genuine? Amazon is awash of people selling Dell windows DVDs with the COA stickers for around £40. There's the odd advert for a home premium windows 7 for £40 or so also.
With the news that windows 10 will be free for windows 7 or 8 users is that a sensible way to go? The OEM versions are motherboard locked I udnerstand but home premium is presumably deemed a full version and able to be transitioned should I upgrade hardware.

Many thanks

[Golaboots]

Thanks for the replies. I too find there's nothing inherently wrong with Vista itself.
What is disappointing is that many new software packages don't support it. Lightroom 5 for example.