The borax will work, though you might need to use it a couple of times. Put some socks and slippers on and let it hang out in the carpet, use a brush or a broom to get it in deep in the carpet. if you're still having problems after that, flea bombs should kill what's left.
How long do you think I should leave it 'soaking' in the carpet, as it were? Are we talking a half-hour, or longer?