Windows utilizes many different .wav files int the OS, and most of them are at different sampling rates and bit resolutions than CD quality .wav files. Disable all of those nifty windows sounds and I am sure that the problem will go away. If your soundcard has a preference for playback, set it for the bit resolution and sampling rate you want. As long as no other file comes along and changes it, you will not have any problems. I get this same thing sometimes with Media Player, it doesn't seem to make the sound card play it's files at the saved sampling rate.
Good luck.
Ed