Floating desktop works with XP only, if I remember correctely (don´t have any vista or win7 experience)
Go to the screen where you set your resolution
select enhanced or extended settings (I have a german windows, so I don´t know the exact wording, but it is the lower right most button )
select Monitor
there is a checkbox which reads "Modi ausblenden, die von diesem Monitor nicht angezeigt werden" Something like: don´t show modes this monitor does not support.
Uncheck this box
You can now set resolution to anything your graphics card can handle. If this resolution is higher than what your monitor can display, whenever your mouse pointer reaches the edge of your screen, the whole screen will scroll in that direction.
Hope this helps.