Almost every legitimate drum pad has MIDI support, which you can easily route to the computer via any cheap $20 sound card with a game port. (Alternatively there are also USB MIDI adapters.)
Then, if you're using MIDI, you'll find millions of sampler programs that will happily accept a MIDI signal and spit out sounds. No programming or custom hardware required.
Drum "pads" are often meant to be used with drumsticks, but a lot of drum "machines" or "MIDI Pad Controllers" just have little basic buttons to tap with your fingers. You might find something cheap on eBay or Craigslist.
This one's expensive, but it'd work well -- it even has a native USB interface:
http://www.musiciansfriend.com/produ...D24?sku=700812
Rock on.