You almost certainly have a ground loop issue. The ground on the KRKs is at a different potential to the ground on your PC.
A ground lift switch or isolator might work for you. As may plugging the PC and active speakers into the same wall socket.
A more permanent solution is difficult to recommend at your budget. I don't know of a USB interface with balanced out for $70 or less.
If you could scrape up a little bit extra you could try this M-Audio M-Track for ~$100.
You will also need two new cables. Balanced 1/4" jack to XLR. Most interfaces use these jack plugs rather than XLR to save space on the back panel. Same thing though from your point of view.
The balanced cable from interface to speakers provides isolation. (No common ground). Rather like (but better than) how some people use optical S/PDIF between audiophile DAC and PC tp break the ground connection.
Since you have bought a pair of prosumer active monitors it makes sense to also buy the sort of interface they were designed to work best with.