It is hard to imagine that player software does make a difference.
You select a song an it will play using the audio drivers supplied by the OS.
We most assume more esoteric effects like some players are more frugal on resources than other and using more resources affect sound quality in one way or another to explain possible differences.
However there a couple of reasons why there might be a difference.
WMP and I think iTunes use DS (Direct Sound)
A player like Foobar can use WASAPI on Vista.
This bypasses the mixer in Vista as it talks directly to the sound card.Vista Tweaks
It is possible that using a different audio driver makes a difference in sound quality.
A document by dCS explains some of it: http://thewelltemperedcomputer.com/L...ampleRates.pdf
If you want the excellent GUI of iTunes and the configuration options of Foobar, you might have a look at J River Media Center: J River