My screen goes blank (or black) when I switch to seamless or full screen mode, and if I switch it back to scaled the screen remains black.
In full screen, the toolbar (or taskbar) that appears at the bottom of the screen with the menu when you mouse at the bottom does appear, but that is it.
I tried auto adjusting, resizing, minimizing, maximizing, opening settings, closing settings, and everything I could think of, but the screen remains black. In the settings "preview" window, you can still see the windows deskop. It is not blank there, and it is kind of strange because you can open and close windows (sort of) just by eyeballing the tiny "setting" preview screen, but the main VM XP window remains black or blank.
The only thing I can do is power down the machine, and then reboot.
Host is Ubuntu 11.10 with Nvdia Driver activated, and Guest is Windows XP Pro. Guest Additions are installed and appear to be working, or at least the mouse integration is working.