How do I take a screenshot of my desktop?

Press the Print Screen button on your keyboard. Then start Paint (Start > Programs > Accessories > Paint) and press Ctrl + V to paste it into Paint. Afer that you can save or edit the image if you wish.

If you want to take a screenshot only of the active window (say the program you are working with) then press Alt + Print Screen and repeat the procedure.

My computer clock gets automatically synchronized, how can I prevent this from happening?

Under Windows XP:
Click Start -> Control Panel -> Date and Time -> open the Internet Time tab -> uncheck the Automatically synchronize with an Internet server check box -> click OK