The Print Screen key [PrtScn] takes a snapshot of the entire computer screen and copies it to the Windows Clipboard. Hold down Alt and press the Print Scrn key to capture just the active window.
So far so good. Now try this combination - Press Alt, Shift and Print Screen together. And don't worry, you can always restore your previous desktop theme by pressing the same keyboard shortcut again.
Here's what happened to our desktop [before and after screenshots].
This is no XP Easter egg but a resident High Contrast XP color theme designed for people with vision impairment. [via Web Stuff Scan].
Related: Office Pranks - Freeze any Windows Desktop