New cpu = new os? Pls help pc noob

No, you won't you'll have to get a new OS if you change SSD or HDD. But i'm not the biggest expert here so it might be your motherboard.
But a quick answer NO YOU DON'T need to buy a new operating system if you change a CPU.
 
No need to re-install the OS, but if you would re-install, it will likely fail to activate. You will probably have to contact MS and explain you swapped (faulty) cpu. They should allow you to re-activate.
 
Which motherboard or prebuilt PC do you have?

Which operating system do you have?
Was it preinstalled or did you buy it separately?

Generally you don't have to buy a new OS when upgrading your PC. Sometimes you will have to reactivate Windows by phone, if a popup shows up.

Reinstalling is recommended because some configurations are made for the old CPU and could be bad for the new CPU. Blue screen or freezes or not good performing system could be a result of it. Anyway, it should work without problems in most cases.

Backup critical data before upgrading,
Sometimes a motherboard Bios update is necessary to be compatible with the new CPU.
 
Euhm... you should REALLY reinstall when changing to a DIFFERENT TYPE mobo. Chances are, if you are lucky, you will be able to boot into windows. But even then you should better reinstall as windows will make a mess with drivers.

anyway TS is not changing mobo, but CPU. Not even said if its a different type. As long as its same 'family' and same number of cores he should be fine without reinstalling anything.

auto activation will likely give issues when he would reinstall later on, but you should be able to activate by phone.

short answer: NO 😉