Honestly, you shouldn't even be considering XP for any machine you will connect to the Internet. XP is no longer supported by Microsoft, and there are critical in-the-wild exploits for it that you cannot get patches for. XP is also a bad choice from a security standpoint for other reasons - its last notable update in terms of security features was almost precisely 11 years ago, so it's missing all the modern security features - but it was at least a justifiable choice while Microsoft was still patching it.
Just how much RAM do these machines have? While Win10 can technically run on under a gig, I wouldn't recommend it (certainly not for gaming). For 1-2 GB, Win10 (or Win8.1) does very well in general, though it's still underpowered for a gaming box. For more than 2GB, you'd be a fool to put XP on there anyway; XP's memory manager was designed for a time when 256MB was a lot, and it does not make efficient use of large amounts of RAM (remember, unused RAM is wasted RAM. Newer Windows versions use free RAM to do things like pre-cache files they expect to use soon).
Of course, using anything with less than 2GB of RAM to run modern games is a terrible idea anyhow. LoL might run in 1GB if you turn the settings down enough, but it won't run well on so little RAM, regardless of the OS.
2If the machine can run Windows 8.1, then overall, your performance will be better on Windows 10. It isn't a question of anything else other then, can you machine even run it, a Pentium 4 is SEVERAL years older then 5 years by the way ( more like a decade ). – Ramhound – 2015-09-02T00:26:13.717
1I get that. I was given these pcs. I have got Ubuntu running as a server running perfectly on one of them. I don't understand the hate. I understand it's older than 5 years old. In fact the processor I have is roughly 11 years old. Doesn't mean it's out of use. – Mennims – 2015-09-07T09:52:18.910