Horrible co-worker left their computer logged on whilst going to the loo?
Right click on the desktop background, choose properties, and then the appearance tab. Quickly work through all the options, changing every foreground and background colour to white, and change the wallpaper to white. You should be able to change text, background colours, highlight colours, et al. They won't have a clue how to change it back.
( Awesome Welles petitioning for foetuses to be rebranded 'wombles', Thu 12 Jun 2008, 21:42, Reply)