Ok, this is a very strange problem.
I have a dual-monitor setup at my workplace, and recently shifted one of the monitors to vertical position, like this:
Now, I use Ultramon to display different wallpaper on each screen. Basically, behind the scenes, Ultramon creates the following image file, and sets it as a 'tile' wallpaper:
(The two shades of gray represent the two images that I've used).
Now, here comes the strange bit: when I lock my computer (or even just press Ctrl+Alt+Del), Windows forces the upper-left corner of my wallpaper to align with the upper-left corner of Monitor 1, so instead of seeing the two images on my two screens, I see the bit of white image on monitor 1, and the lower part of the image in monitor 2 is cut (because that image is 'pushed down').
I have tried disabling Ultramon completely, and as far as I've discovered, this behavior is inherent in Windows XP - the operating system aligns the wallpapers differently when I work normally, and when I go to the Alt+Ctrl+Del menu.
Does somebody know this, and knows how to circumvent this? currently, I've changed my setup so that Monitor 1 is vertical, and Monitor 2 is horizontal, so the problem doesn't happen. It's also possible to bypass this problem if I move Monitor 2 downwards, so that its upper edge is aligned with Monitor 1, instead of its lower edge - but I can't work like this.