6) hide icons
7) hide taskbar
How do these?
There's a number of ways. For the taskbar, one way is to set to auto-hide (although the real one will pop up when you get the mouse down there, if the victim is still trying to click on the background-image version of the Start Button/whatever he may not activate it). Another is to make sure the taskbar is not locked, then just drag it down to zero-height. (For bonus points, move it to another edge of the screen, first, then drag it to zero height/width.)
To easily undo (if your prank isn't intended to be so troublesome, or at worst reversible by you in a "What was your problem again?" slight-of-hand way), I'd take the genuine shortcut icons into a handy spare folder somewhere, and under the Desktop advanced settings untick the My Computer/Documents icons (or equivalent). There's often some 'special' icons for Internet Explorer or somesuch which might (depending on Windows version) need dealing with otherwise. You could, of course, just move them (and everything else) so that they're just off the edge of the desktop (somewhere where they'd not be easily noticed, against the background), as you could, in the first place, with every
other icon. (You may need to unset any Align To Grid and/or Auto-Arrange settings.)
If I had time, I'd consider setting up a clickable mapping on a .htm containing just the false-screen image, set it as a Web Page/Active Desktop desktop. Having made the clicks on the various bits of the image that should be clickable link to something other than what should have happened (or to open a "hahaha.txt" document, or something), it wouldn't merely be
unresponsive, but be responsive in unexpected ways. There's other tricks I could incorporate into that, but it's been a while since I've even considered such tomfoolery.
(And I've deliberately not mentioned a couple of other methods that are a bit more destructive. A prank should be reversible. You can probably do some .reg manipulation to automate some of the above actions, but I'd make sure you have exported the original settings so you can reinstate them. I won't, but I reckon I could get a batch file together to do most of the work, save for getting the original screenshot. And I think
that, on top of everything else, could be done with whatever generation of more advanced windows scripting tools you might have at your disposal on the platform concerned.)