Is this happening on the latest dev branch? Or just 2.50.3?
I'm pretty sure it's just Windows being Windows. Or WinXP being WinXP, specifically. Most games/programs do that when windowed rather than fullscreen.
Far as I know, the entire tearing issue happens because, despite that you may set a monitor at 60 or 75 or 144 Hz in the display panel, Windows continually spams the screen with as many FPS as possible rather than adjusting its output to the Hz setting. So any program that displays through a general Windows interface gets that problem, because even if that program runs at an even 60 FPS, the fact that it's windowed means it's being translated to the standard Windows FPS rate, and the clash when the frame rates don't perfectly align causes the tearing. Something like when you record a CRT display with video camera.
Of course, when you run a program full screen, that becomes the sole source of the display's framerate, hence when running ZC full screen, its precise 60 FPS aligns with standard refresh rates and eliminates tearing.
The exceptions are programs that run their own individual displays separate from the standard Windows display. For example, movie files that play in VLC or Windows Media Player, like DVD or Blu-Ray rips, have some kind of separate display module. I don't know exactly how it works, but I know that if you press "print screen" and try to paste into an image editor, you'll get a big black box where the video should be displaying. You can also see the video image lag behind the window image if you drag the window around rapidly. They're two separate things, and because that video display is somehow independent of the standard Windows display, the low FPS video divides perfectly into standard refresh rates so you never get any tearing, whereas window-based streaming like Netflix will get tearing in fast action/panning scenes.
Unless you're going to somehow do like the video players do and make ZC have its own separate display module beyond being a basic window, I don't think there's anything you can do about this.
(And yeah, I have no idea what's up with dev branches.)