Just be aware of the risks
If you are using a Win XP and it works, or you can aquire a working PC with an older OS, just be aware of the risks. These are not like old vacuum cleaners or bicycles or food blenders that as long as they are working they are just as useful as the day they were manufactured. Those are self-contained machines. They don't interface with anyone else by you. But as has been stated, PCs are a different animal. If you choose to use a PC as a stand-alone piece of equipment to do things like WordProcessing, storing your photos or music, or using it to do your accounting, AS LONG AS IT'S NOT HOOKED TO THE INTERNET, it will work beautifully and flawlessly for eons, or until it's moving part -- the Hard Drive -- fails.
But most people are driven to use the PC to communicate and share with others if nothing else, the all important email. For that the PC must be connected to the Internet and therein is the safety issue. The internet....think of it as a vast wasteland populated with 45% pornographers, 30% theives whose sole mission is to get into your computer to steal everything you own including your identity, 10% businesses whose sole driving purpose is to separate you from your money...legitimately perhaps, but your money will be gone, nonetheless, and then 5% unverifiable misinformation and lies, and about 2% actual, useful information. Given THOSE percentages, you really need to rethink how useful that old PC is when it isn't updated with the latest safety upgrades to protect it from that host of evil which continually seeks to attack it. And if you think computers are not constantantly under attack...
At a major college IT department here in NYC, they installed very sophistocated monitoring firewall systems which would alert the staff whenever a hacker was detected trying to breach the firewall and other protections built into the system to protect the network. As soon as they turned it on, the alarm, lights and sound began to show attacks. After a day of this, they had to at least turn off the audible alarm because it was driving everyone nuts. They get hundreds of hits a day. Of course that could be less for a single home user, but no the less, thing of your PC's network card as a beacon that is constantly shouting out your IP address to the entire planet. It's saying "Here I am....Here I am." The world KNOWS it's there; it's just a matter of time some nefarious hacker decides to respond, "Here I come...Here I come."
Seems to me, when making the decision to use a non-supported (think non-protected) PC, just be aware of the risks you assume the minute you plug that network cable into your service provider's modem. OK, use it for simple stuff, but I certainly wouldn't use it for on-line banking or accounting or any thing that contains banking or accounting information. In fact, I wouldn't even use it to buy anything on line because that requires you to input just too much information and if somewhere along the way a trojan virus has silently gotten into your computer and some felonious ******* is out there recording your keystrokes and passwords, you can kiss your account balances goodbye.
I won't buy anything on line unless I can use the proxy credit card that banks like Citi have which create a unique card number for each sale. Once the merchant uses that number to complete the purchase, it becomes invalid AND you can make the balance match your purchase exactly so that the merchant cannot charge you a penny more than the purchase price. It dices and slices those "recurring" billing practices. Just let them TRY to bill me again next month....HA!!
All I am saying is, you really need to be cautious when on the internet with an older machine. Hackers know the odd that the majority of people DON'T update the OS or anti-virus protection regularly. They plan on those odds being in there favor. Imagine how happy they are to find millions of Window OSs which haven't had security vulnerabily holes plugged in decades still connected, still broadcasting their IP address beacon.
I keep a link on my desk top to XPs Network Connection. You can right-click on it and DISABLE the connection with a mouse click. Whenever I am working on the computer and doing stuff that doesn't require an actual internect connection, like composing a letter or email, playing a game, or anything that is stand-alone, I simply disable the connection -- think of it as switching off the outside world and all who would try to get at your computer. When I need to connect to the outside world, it is just a click away. I know people who leave their computer on DAY AND NIGHT connected to the internet. Silly, silly, silly.....IMHO.