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].

[0]: https://item.taobao.com/item.htm?spm=a21m2.8232368.0.0.TlUMn...

Don't most manufacturers pre-image OS on computers at the factory now? They usually have an option to create recover disk/sb drive.

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.

Holographic stickers are routinely counterfeited in China: http://www.bunniestudios.com/blog/?p=283

Last time I had to buy Windows install media I went directly to a Microsoft corporate store.