Check out the Unity engine. They've done a lot of work on making their tech work with a very wide range of hardware, and have nice tools like graphics emulation so you can see what an older machine would be drawing while on a newer machine. It's relatively cheap and has a really good workflow, so as long as you're not trying to squeeze all the performance you can out of an older machine it's hard to beat it's price/value ratio.
Fundamentally, though, if you're dealing with old PCs, you're going to be limiting yourself to something probably DX7 level or so. OpenGL drivers are notoriously bad, so I'd suggest going the DirectX route. You're probably not going to have pixel shaders (those came out with DX8/9 cards I think), so you need to do everything fixed function. Unity handles all of this for you.
Edit: You can download an use Unity free of charge, with certain restrictions. It doesn't play well with team development, but if you're doing small things by yourself then it's more than capable.