At that price, you pretty much are limited to refurbs and you can forget about laptops with DVD players unless you want something with ancient specs. Your options vary a bit depending on where you are so I can’t get too detailed, especially since us Seattlites have options that other places don’t.
Personally, I got a Gateway desktop for about $500 with no monitor (I used my old one) or webcam (I don’t need one) and it as served me well thus far. I also got a mid-range Toshiba laptop for $500 and an external DVD burner for another $70.
You might be happier with lesser machines than what I got, but you can’t really get much cheaper than I went unless you want something that will need to be replaced next year or manage to find better sales than I did, so spend the extra $100 and know that you will get twice the computer for only 25% more money.