Getting widescreen when your primary is 4:3?

[Objekt]

I have a somewhat unusual display configuration, with an old-skool CRT monitor (4:3 aspect ratio, run at 1600x1200) as the primary and an LCD (16:9, 1920x1080 native rez) as the secondary.

Virtualbox doesn't seem to want to let either my Windows XP or Windows 7 guest VM's run at the native resolution of the LCD.

I tried dragging the Windows XP VM window over to the second display, then selecting "Fullscreen" mode. Indeed, it went to fullscreen mode...back on the primary monitor!

There is no way to select 1920x1080 as a resolution in either VM. The choice simply isn't offered.

I found no recent discussion of this problem anywhere else. I seem to be in a unique situation.

[vbox4me2]

Play around with the xml file for this VM, ea:
<ExtraDataItem name="GUI/LastWindowPostion" value="260,23,800,640"/>