I started to send some emails this afternoon, but when I typed the character 'd' short cut to desktop the same with 'm' to my computer, what have I done? How can cancel it?

Sounds like you may have been mashing the windows key while typing or the windows key was stuck.

http://support.microsoft.com/kb/301583

• Windows Logo (Display or hide the Start menu)
• Windows Logo+BREAK (Display the System Properties dialog box)
• Windows Logo+D (Display the desktop)
• Windows Logo+M (Minimize all of the windows)
• Windows Logo+SHIFT+M (Restore the minimized windows)
• Windows Logo+E (Open My Computer)
• Windows Logo+F (Search for a file or a folder)
• CTRL+Windows Logo+F (Search for computers)
• Windows Logo+F1 (Display Windows Help)
• Windows Logo+ L (Lock the keyboard)
• Windows Logo+R (Open the Run dialog box)
• Windows Logo+U (Open Utility Manager)