Upgrading my Pc that came with Windows 7 pre-installed. Can I re-use the licence or am I expected to torrent buy another?

you can use the same if its not a oem license

It’ll be an OEM. Am I screwed?

Depends 🙂

If it’s a Dell / HP etc.. maybe – you’ll want to reinstall onto your new hardware – at which point the disc will kindly point out that it’s not the appropriate brand. If it’s just a generic disc (and not tied to a given manufacturer) then go for it. Legally; if it’s an OEM copy you may be in a bit of a grey zone – morally I think you are in the clear (as long as you aren’t handing on your disk / copy for someone else)

You might have to ring a number to re-activate; it’s usually an automated line; I’ve occasionally had to talk to someone but have never been questioned beyond the ‘why do you need a new activation’ – ‘because my old box died/exploded/I got bored and bought some new bits’

Just do it; and if it all goes wrong install Ubuntu and then spend 3 months wondering how it all works…why you paid microsoft in the first place.

If it’s the same machine and you’re just changing some of the bits inside, that’s fine. It might ask to be activated again, but should go through fine. Worst case you have to ring up, say you’ve changed hardware and they’ll allow another activation.

Officially, you need a new one. In practice I’ve reused OEM licences about 4 times. It seems to help if you tell them an old part failed and you couldn’t find a replacement the same, rather than “I got a new processor so I can play Skyrim on maximum” or similiar.