I had this problem too at one point. The best thing to do is to stop everything and restart your computer. This will stop all Ethereum processes. You could kill processes using the task manager / command line but this will be easier to do and it will ensure you do not kill important processes not related to Ethereum.
Once you restart, re-run the GUI. If you still get the same error then I suggest (PLEASE ONLY DO THIS IF YOU DO NOT NEED YOUR CURRENT ETHEREUM ACCOUNT IN CASE YOU INCUR LOSS OF DATA OR FUNDS) that you uninstall Ethereum. Then go to your folder C:\Users*YOURUSERNAME*\AppData\Roaming\ and delete the Ethereum folders. You may need to restart you computer again before you can do this to ensure no ethereum processes are running after you tried to re-run Ethereum.
Once this is done, re-install Ethereum, let it resync the blockchain and then you're good to go. One tip... never close the ethereum GUI using the X button, instead click 'Ethereum' and 'Quit Wallet'. This will end the processes without leaving them running in the background and locking your folders.
Hope that helps!