Opening Windows "Nudging" Avatar

I've never seen anything quite like this... I noticed this morning that when I open certain windows on the left side of my monitor, my avatar is being nudged as if I had lightly tapped the right arrow key, or as if someone bumped me.

For instance, I open my Inventory window, then click File>New Window, it occurs.

I didn't notice this last night after I installed the new 1.20.16 security update, but I probably was not opening a lot of windows then, either. But this morning it has been really noticeable. It took me a while to figure out what was causing it. I thought I had a hardware problem with my keyboard until I noticed that it only happened upon opening windows to the left of my avatar.

No hardware or software configuration changes here. Running XP Service Pack 3 with a NVIDIA GeForce 8800 GTS on a single monitor. Nothing unusual or unique about my setup that should cause problems.

Has anyone seen this before?

I see that little nudge every time I open or view a notecard. It's been like that for me from the beginning. Not sure exactly why it happens, though.