Most (all?) equalizers plug between source (your PC) and destination (your speakers). So, as long as the equalizer you choose has standard stereo input / output, you can plug it. You might need a couple of 3.5mm-to-RCA adapters, depending on the equalizer' connections. Here is an example where you'd need these adapter cables.
No reason you can't use software EQ. Much more flexible and free too.
EQ can be used to make improvement but you can't go nuts adding a lot of boost. It causes distortion and can damage the speakers.