That's great. Now, what brand and model ADSL kit and what type of laptop? It's hard to suggest modifications to hardware if we don't have the slightest clue as to what you have to work with.
You should be able to go to about 150ft if you have line of sight with the typical laptop. However, that assumes line of sight. If there are some obstructions, such as walls, in the way, the range will be much less. For your 20ft test, is there anything in the way?
Another possiblility is that someone set the wireles speed to54Mbits/sec instead of "auto". At a fixed 54Mbits/sec, your maximum range will be about 15ft before the error rate climbs sufficiently to make the connection unuseable. Check your settings or go back to the defaults.
Yes. The usual answer is external antennas or bigger antennas on the unspecified router kit. However, in this case, if you're only getting20ft of range, then methinks something is broken. It's difficult to tell whether it's the unspecified wireless router or the unspecified laptop. The easiest way to tell is to drag the laptop to a free wireless hot spot (coffee shop) and see how it performs in a known working environment. If it works in the coffee shop, then the wireless router is the problem.
You can do a few things to improve range. For example, slower speeds have greater range because the receiver sensitivity improves. If you change your wireless speed from "auto" to the slowest OFDM speed, 6 Mbits/sec, you'll get a noticeable range improvement, at the price of speed. However I wouldn't bother until you figure out why you can only go 20ft. It may be nothing more than a misconfigured wireless router. If you had fixed the speed at 54Mbits/sec, the maximum range is about 15ft, which is roughly what you're getting.