Musichal, DaveVoorhis, and so many others in this thread have given you good advice about what can help you get decent sound out of your computer. So let me add to that, and also say that I'm using exactly what they're recommending: a little 'chip' amp.
Here's a link to one that's actually on sale for only $20, which is a really good price:
https://www.parts-express.com/lepai...-stereo-amplifier-with-power-supply--310-3000
You have said, more than once, that you have bookshelf speakers lying around. Depending on their efficiency, these little Lepai amps might surprise you. I'm using one with my Mac to power a pair of Klipsch bookshelf speakers, with no feeling of it being underpowered. It is a
vast improvement over any computer speakers I've used in the past.
Lastly, as someone else also pointed out, you frequently mention how broke you are; yet you seem to often pony up small dollars for junky stuff at the thrifts. Ante up a measly $20 while the Lepai is on sale, hook it to your sound jack on your computer, and then hook your bookshelf speakers to it. All that other stuff can go back to the thrifts.