how to develop an active desktop application in c#

i want to develop a nice thing that will be on the desktop (like an active desktop) something like this:

http://www.tinnes.org.uk/desktopcalendar/about.htm

or this:

http://w开发者_JAVA百科ww.xemico.com/adc/screenshots.html

Edit:

That gadget should be common for XP,Vista, Windows 7 and that gadget size should be adjustable

If you're using vista or seven you likely want develop a desktop gadget.

Look at this answer for some hints: C# tutorial to write gadgets