Well, you've got no external antennas on the router, so you'll either need to move the router to a more central area, or play with your USB wifi adapter. Since it's on a cable, many different configurations might be possible. High as possible and with as little around it as possible. Older homes, with screen mesh behind the plaster can really dull a signal.
There are 'boosters' so to say, but any wifi extender is going to cost more than another router, often times routers can be configured for WDS. WDS is a repeater mode which links to a 2ndary router to extend coverage. It's not terribly fast, since it's receiving and then repeating everything that needs to go through it, basically cutting speeds in half.
Try setting the upstairs router on the floor (unobstructed if possible) might help penetrate to the basement. You could also try a metal object on the back side of your USB setup, to kind of 'reflect' it's focus towards the router.
Or, poke a hole in the floor, run an ethernet cable, and be done with it.
(Sounds scarier than it is).