If you don't know what parts you have in your computer you could run Speccy on it.
Speccy
https://www.ccleaner.com/speccy
Somebody gave me an old laptop they paid $0.50 for at an auto junkyard. I salvaged the 250GB 2.5-inch hard drive and use it for backups for my old laptop. Before I reformatted it I scanned for and got the product keys for Windows 7 Home and Office 2007 Standard. I used the product keys and used them on another computer.
I built a desktop computer in 2002. I installed Windows XP Pro on it. In 2012 I upgrade the RAM to 2GB and upgraded it to Windows 7 Home. It is slow with Windows 7 but at least it works. I did the same with the laptop I bought in 2004 that had Windows XP Home. I upgraded the RAM to 2GB and upgraded it to Windows 7 Home.
It is OK to play around with old computers but if you want to use them on the Internet they need to at least have Windows 7 on them. If you don't have the money to buy a more recent version of Windows you can always install Linux on it. The biggest advantage of Linux is that it is free.