Discord Messenger is a free and open source Discord-compatible chat application. It targets Windows, specifically Windows 2000 and up.
The main project can be found at [https://github.com/DiscordMessenger/dm]. Its constituents are linked as submodules within the main Git repository.
The official Discord client is notorious for using a lot of system resources, and oftentimes these old systems can't even get the chance to run the official application, simply because of the high requirements.
Instead of trying your darndest to upgrade your scrappy old computers to Windows 10/11, why not use an unofficial client for Discord that actually works on your computer already?
As the developers of Discord would put it, It's time to ditch MSN and Yahoo.