I think I may know why this problem is occurring.
To test you would need two audio files, one for which the “slowdown” playback problem occurs, and another for which the problem does not occur.
Check the sample rate of each song. To do this in XP, right click on the mp3, select Properties -> Summary -> Advanced, then scroll down to the entry for “Audio Sample Rate.” Post back the numbers here.
If it is what I suspect it is, then one mp3 will probably have an audio sample rate that is integer multiple of 44.1Khz, and the other will be an integer multiple of 48Khz.
