Try booting to the Live CD and seeing whether the problem exists when booted to it. If it does not, that indicates that something went screwy with the install.
Also, you could try running xev
in a terminal, then press a key before pressing the menu key and after (xev captures every mouse movement and keyboard press, so practice a little before doing the capturing). Capture the output and post it here. If you do that, please be sure to click the "Go Advanced" button below and put the copied output inside of "Code" tags.
I think that xev is one of those little-known utilities already in the software load.