I know, you hate to throw out good hardware, and there's merit in keeping the old stuff alive. But seriously, look around. You can get refurb Core2Duo 3.0Ghz machines with 4GB of RAM and a small HDD, WITH OS, for $50-70 if you look carefully and check out Newegg's online sales, when they have a refurb sale.
Or look at ebay, you can get SFF models of older business-class Dell units, fully-functional, and most importantly WITH A GENUINE INTEL QUAD-CORE CPU, 2nd- or 3rd-gen i5, for as low as $110-130, if you look around. Again, generally, WITH OS, (or at least, WITH COA, and with the proper installation media, you can re-install the OS).
The modern web is rather script-heavy (use an Ad-blocker, or several), and you really owe it to your folks to get them something non-crappy. Those older business quad-cores, have quite a bit of grunt left in them, and they're hella cheap, because they're everywhere.
I mean, you could pay Newegg $300 for one, in a nice package with a keyboard and mouse and a one-year warranty, if you prefer that route. But if you're into ebay "adventures", and don't mind installing your own OS copy, then you can save mucho coin.
I would get one of those quads, with or without OS (but with COA), and install a decent little 120GB SSD, and re-install the OS on that. That will be like a rocket to them, and you may never have to buy them another PC, ever.
Edit: If you do decide to go the P4 route, then yes, definitely install some flavor of Linux. Linux Mint 18.2 Cinnamon is popular. Depending on which P4, it may or may not be able to take a 64-bit OS. If it CAN, then go that route.
Don't waste a good copy of Windows 7 on a crappy old P4 rig. Plus, if it's PATA, it could be a pain to stick an SSD into.
Like I said, check out Newegg's 2nd-gen or 3rd-gen quad-core refurb i5 rigs, or ebay. Look for i5-2400, i5-2500, i5-3330, i5-3470, i5-3490, etc. Don't worry about the drive size too much (and don't overpay for what will likely turn out to be a server refurb 2TB HDD, if you get one that big, with bad sectors and years of runtime), because you'll just put a new SSD in.
Edit: Yes, I have some experience actually doing this.
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