Any platform with internet and USB support - even Mac or linux - can be used to download a Windows OS image (from Microsoft's site) onto a flash drive. You may be able to use it for some period (maybe 30 days, 90 days, or whatever) but at some point a product key will be required to continue use.
You cannot skip the Microsoft product key component during a Windows install. Aside from resorting to piracy/hack methods which are, of course, illegal and may not work for long (if at all) anyhow. Legal and ethical issues aside, installing a pirated operating system found somewhere in the darkest, vilest pits of the internet is a sure way of setting yourself up for all manner of OS-embedded rootkits, trojans, keyloggers, and nasty malware payloads which will cause endless headaches.
You should have a legit Windows key somewhere on your machine anyhow, usually on a sticker affixed to the rear of the chassis or perhaps underneath a removable battery or drive compartment.
You might be able to install the Microsoft Technical Preview (Windows 10 beta) and run it for a while. I believe it expires/deactivates in just a few weeks - but it might be enough to get your machine running, and (who knows?) maybe you'll even like it enough to buy/register your copy since you're in the market for an OS anyhow.
Personally, I always purchase a real bona-fide genuine OEM Microsoft Windows CD/DVD, no downloaded "cloud" installs, no licenses, no computer shop, no third-party rebranded junkware builds. It allows me to install and reinstall and migrate my copy of Windows all I want across any machine I want until (and even beyond) end-of-life retirement for the OS version. Having a dependable, clean always-working bootable disc pays for itself in saved time and effort. I highly recommend this approach.
Also worth becoming familiar with a non-Windows OS (ahem, think Tux). For when Microsoft's corporate greed becomes unworkable. Or for when you need a backup/redundant OS. Alas, the sad truth is that if you run games then you run Windows.
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