What kind of music do you listen to? If having a ton of bass is not the most important thing, I would recommend looking to get a higher quality pair of speakers and trying them out on their own first. A good pair of speakers is going to be the most important element in how things sound.
Computer speakers really need a sub to cover the low end since the tiny satellites typically can't do anything resembling bass on their own. The physical size of "real" speakers and their increased driver area really helps them out in that department.
I know what you mean about that desk being heavy. It was enough work to just move it a foot away from the wall to give the speakers some room in back
https://mywebspace.wisc.edu/spmclaughli...k%20System/X-LS%20side%20and%20top.JPG
Having a receiver already really simplifies things. As long as that's in working order, you'll be in great shape using that.
To connect to your computer, you'll just need to get an adapter like this
https://www.cablesforless.com/index.asp?PageAction=VIEWPROD&ProdID=14203
You'll plug into the 3.5mm stereo output of your soundcard to one of the inputs on your receiver. (Aux or whatever it has) Then just wire up speaker wire to the speakers paying attention to get the polarity right.
You could certainly go with a pair of $50 speakers like the Polks and a cheap sub like that Sony or the 10" Dayton from partsexpress, but you might want to think about just getting the best pair of speakers you can afford instead and worry about a sub later.
It would be a lot easier to just add a sub in later if you thought you needed it vs. getting very inexpensive components and possibly trying to upgrade it later.