Terminal computing doesn’t require much computing power of the clients. With minimal time investment, your old PCs may be able to be converted into wonderful Remote Desktop terminals.
It is probably safe to assume that your old PCs have an obsolete Operating System (OS) that is no longer secure. So, the first step is to ditch that baggage and get an LTSP server going so that you can boot your client terminals straight over the network.
First confirm that your old PCs have a “PXE Boot” option. You likely will have to enable this feature in the CMOS/BIOS, and then adjust the “boot sequence” so that the network/PXE device is first. If you do not have a network/PXE option, then the machine is probably not capable of booting to the network. While you can still install an OS on the existing hard drive, that — in our opinion — is not a worthwhile option. You are likely better off scrapping the machine and moving on to the next.
With the LTSP server in place, and the client configured for PXE boot, you no longer need a hard drive in the terminal. We recommend you remove the hard drive and securely store or dispose of it. While you are under the hood, disconnect any other unnecessary drives (CD/DVD, floppy, flash) to further reduce power consumption.
Upon disconnecting unnecessary drives, you may need to enter your CMOS/BIOS setup once again to disable those devices. Test that your client boots-up to the network and logs-in to the LTSP server. Repeat the process for your other obsolete desktop PCs, and soon — for no more than the cost of your time — you’ll have a fleet of lean, up-to-date, easily-managed terminals.