The CAD key effect suggests to me there's something software going on.
I also do not offer any solid solution other than a complete wipe and install. If you are willing to try Grif's advice then I'll supply a link but once in a while I encounter folk that want the assured answer. That's a bit costly of our time but there is a sure fire fix.
I'd try Grif's scans next if this was my PC.
I've been dealing with this issue for almost a month on a Dell XPS 17 (L702X). Regardless what programs open or what mouse is being used, the mouse sporadically will stop responding to select buttons or highlight when I hover over them. Sometimes it will allow me to select items on all or part of one window but none at all on others. I found that pressing Ctr Alt Del will immediately get the mouse to respond correctly but that is short lived and it will stop again without notice. This may continue over a period of time, and then pc goes back to normal just like that. I tried other mouse, changed the batteries, update the drivers, and ran different virus and malware scans which all came back clean. I ran hardware diagnostics and all is fine, no issues in device manager and nothing show in system logs. I even went as far as performing a complete clean install of Windows 8 a few days ago. Yet the problem remains. I see a lot of forums with similar issues but no one has a solid solution. It's similar to the issue of the monitor going black for a few seconds. I'm at a complete loss on this one. Anyone else having this problem?