go to my computer,
Tools-->Folder Options-->in folder options you can see first tab as General,
in that option you can see third option below as "Cick items as follows",
If it is selected as "Single click to open item" then select-->"Double click to open an item"
Tell me if it works or not...
and also tell me ...have you tried using another mouse for same desktop so that we can be sure its not a mouse issue...or....try changing the USB port of the mouse