Weird. Could be any number of things, really. Have you tried playing them in any other software besides WMP? Or a different set of speakers/headphones? Does the same thing happen when you play mp3's or cds? Could be:
(a) either WMP or the drivers for your soundcard have gotten corrupted and need to be reinstalled.
(b) a bad or loose cable from pc to speakers.
(c) distortion caused by windows volume too high
(d) you could get similar artifacts if the computer can't keep up with reading/outputting the file fast enough. this could be caused by low memory, a badly fragmented hard disk (or bad cdr if that's where your files are), who knows. this sort of thing really shouldn't be happening on such a recent computer unless something is really wrong. no other problems besides audio playback? no blue-screen crashes or anything?
well, good luck, let us know how it goes...