Why does my PC sometimes think I am constantly pressing the control key?

This could be caused by the 'Sticky Keys' function (an accessibility option). If you are on Windows 7, click on the start button go to Control Panel, ease of access, change how your keyboard works. Under the option 'Make it easier to type', uncheck 'Turn on Sticky keys' and see if this solves your problem.

For me, this seems to happen when some other program intercepts the controls while your using the Ctrl key. In my case if I am working in LogMeIn and I have the Ctrl key down and an email alert pops up, the Ctrl key will be "Stuck" Until I tap it again.

If it continues to be stuck even after pressing it, you may want to run in diagnostic mode by running msconfig, then selecting Diagnostic startup. Start the services and programs you need manually and see if it still occurs.

Ran into another problem exactly like this one, but the cause was completely different.

Go to Device Manager and search for the section named "Keyboards". Under this tab, if there are two "HID Keyboard Device" entries, right-click and uninstall one of them.

For me it it gets fixed by pressing Fn and the Windows key at the same time.

I had a stuck Alt key happen to me once on a Samsung Chromebook using LogMeIn.

I was able to get it unstuck by pressing the "Magnify" Key (above the Shift key).