Nexus 7 can't connect to PC When choosing MTP or PTP, the box is checked, then when I exit the screen and go back to the USB Device Settings screen, my previous choice is not seen, and both boxes are unchecked. It's like the Nexus has forgotten I've made the choice.

Now I can't connect my nexus to PC as the PC won't recognize it! 

I recently plugged my Nexus into my LG TV using the USB lead, if that is at all helpful.
Sometimes you need to lock and unlock the tablet to have it connect. If you are running any sort of task manager, disable it. Can you post some screenshots?