The question isn't "Is Windows installed"... it's "Is the install of Windows on a laptop legal and legitimate".
A sticker says Windows is installed... it doesn't say anything about legality.
OP said "I couldn't find a laptop with a legal copy of Windows installed". Responder said "Everything I got was legal, and had no issues buying new discs with legal licenses".
OP said "it was literally impossible to buy a computer with a legal copy of windows installed on it".
Responder said "it is quite possible (even common) to buy a computer with a legal copy of windows installed on it" and asked "is a Windows sticker on a laptop good proof for legally installed Windows?" with the assumption that pirated Windows does not come with a sticker.
PS: That assumption turns out to be debatable [0].
In Asia most brands sell the laptops without OS for cheaper price compared to the same model but with windows preinstalled. Some stores may install a bootleg version for free or even fake it as a genuine Windows install.
