Probably mostly laptops & desktops. Servers are likely not running Windows, or have already been rotated out to have the newer hardware.
And yes, wiping and reinstalling the OS will make you *reasonably* safe from pre-installed malware.
There could still be nasty stuff in the BIOS/UEFI. That is less common, but it HAS been seen in the wild. That's very difficult to be completely confident that it's been eliminated, but re-flashing the BIOS with the latest version will clean out some malware. There are others that can survive that. It just depends on how confident you want to be (or, put another way, how paranoid you are).