i just upgraded to windows 7 ultimate and have a similar problem... i can right click everything other than the desktop. and actually... it appears that it is thinking about right clicking, but isn't responding right away. like there is a lag on right click options, but only for the desktop. everything else responds right away.
i went thru the group policy editor and everything matches what has been suggested already.
addition: also i cant double click to open the recycle bin. same thing it appears to be thinking about it, and several seconds later it will pop up. like it's lagging behind.