Huh, that is certainly something that shouldn't happen, and it most definitely is not related to resampling (which is applied per sample, not per stereo channel). Random guess: Are you using a DirectSound driver? If so, change it to WASAPI or WaveOut in the sound card options.

http://i.imgur.com/UfIK1J3.png

This is what I see. I'm not sure if I'm looking at what I need to be looking at though ... :(

Oh right, Windows XP. In that case, WASAPI isn't available of course. Was the driver that is currently selected (WaveOut) the one you used before? Please try going through all of them to see if the problem persists. Especially if it persists with the WDM-KS, we can safely assume the driver is the issue and not OpenMPT.
Do you have similar problems with other applications? From the size of the screenshot, I guess you're using some kind of very low-spec netbook with integrated audio?

Yes, yes and yes. The problem seems to be the driver, after all!

No other audio applications have given me this issue, but more than likely it's just this silly old netbook.

I'm not quite sure if anything can be done there, since if the problem happens with all available drivers in OpenMPT, it most likely isn't a fault on our side. Maybe manx has some more ideas, but right now I can't really think of anything that could go wrong. One last thing to confirm could be the output configuration - what sampling rate and bit depth are you using? If it's not 44 KHz 16-bit, you could try using that.

It already is that, as a matter of fact. At any rate, I'm receiving a new computer in a few days (one that is fit to more modern standards, thankfully) and I will report back on the issue (if it exists at all) then. Thanks for your help!

Does using 48KHz instead of 44KHz maybe help?

48KHz actually seems to exacerbate the issue (it happens every time I play when I do that).

I'm totally confused here. The problem happens for all OpenMPT outputs and does not happen in any other application at all, did I get this right?
If it should in fact be related to some broken hardware driver on your system, you might want to try changing the system-wide audio device settings in "Control Panel -> Sounds and Audio Devices -> Volume -> Speaker settings -> Advanced ... -> Performance". I cannot really recommend any particular setting here, just play around with both, the "Hardware acceleration" and the "Sample rate conversion quality" sliders. You may have to completely close OpenMTP and start it again after changing these values. In OpenMPT, try each setting with 44100 as well as 48000 and always use stereo and 16bit (the most standard settings).