Shift and Ctrl with Mouse left button

This happened on occasion in XP also, but now on my new Laptop with Win7 I see it happening again. Every now and again for some reason I cannot select random files (in a file handler) by holding down the CRTL key and selecting desired files with the left mouse button.

Also sometimes the SHIFT key will not let me select the first and last files for a group selection.

And then again on another boot up they both work again until some unknown combination of events messes it up again.

Anybody know what does this ?