What can I do with an old desktop PC ?

As a 'Still Working' retired PC tech, my customers are always giving me their old PC's when I set up a new one for them. I now have 20...... 14 Desktop and 6 Laptop PC's, all running windows 7 or better and all in great shape. I've set up a New User (me) with a fictitious name, like "Joe", on all of them and then deleted the old user which also deletes all the old user's personal files. That's a quick and easy way to change ownership on a PC, without damaging the OS, or loosing proprietary Drivers. Of course I clean out the junk files, clean the registry and defrag the hard drive. Then if anyone needs a replacement PC, CHEAP, I can provide them one at a very minimal cost. They mostly just pay for me to deliver and set up the PC for them. I only send PC's to Reclamation when they have bulging capacitors on the motherboard. Everything else is repairable. Cheers mates! TechnoMage

Do a Clean install of each to Windows 10 while it's still free, then donate them. Most schools won't want them as they have their own IT department and budget but some Thrift Stores affiliated with local churches might be thrilled to get them. I know I would be as I work at my churches Thrift Store. I refurbish donated computers, install Windows on them then sell them cheap (or give them away in some cases). The money then goes to our social outreach center to support our food pantry and provide help with utility bills for low income people.

You could also contact a local school and see if they know of anyone who could use them.

I do the same, but I wiped the HDD and install a Clean version of Windows 10 and some basic software like MS Office etc. And I sell it at a deep discount. Just removing the account gets rid of their personal files, but they may have files somewhere else on the drive as well as software another user won't want or need.