Sorry for my terse response. I'm not sure what you're looking for in terms of digital to analog conversion, and I'm not particularly familiar with "computer" speakers, so I'll leave that to someone else.
I would strongly suggest you drop the idea of a subwoofer for now. You definitely want one, but you can't get a decent sub by itself for under $100, which means you're not even in the game to go 2.1 or 5.1.
I would look at getting the best pair of speakers you can afford. In your price range, you should be able to get a pair of pretty good used speakers. Depending on what you will use to power them, that would be next. If you have a decent receiver, then your next step would be the sub. Once you get a decent 2.1 system going, you could look at adding cheap surrounds for the time being, and eventually a center channel (that preferably matches the front pair).
You don't have to follow that approach, but I can assure you that you won't find any 2.1 setup under $50 that even approaches "barely acceptable" sound quality.
Good luck!