If it's for Gaming, Windows 7 would be your best bet. While not ideal, do keep in mind you're running quite an old system with that kind of Motherboard and any modern Gaming isn't going to run very well, outside basic Esports titles at best. If you're doing general computing tasks you could look into Linux as it's free and while i'm far from a Linux expert, there are plenty of flavours around and many seem to run far better than Windows with lower spec hardware. If you're interested in this route i recommend having a look around online.