1. Go to Safe Mode > select Safe Mode with Internet.
Do you have Malwarebytes installed in your computer?
If yes, please run a full scan.
If you don't have it, here is the download link. Select the FREE version.
http://www.malwarebytes.org/products/malwarebytes_free/
Run full scan with Malwarebytes. See if it catches anything. I think your computer is infested.
If it catches some viruses, remove what it has caught, return to normal mode and see if you have regained your admion rights.
2. If the above does not help......
You said only recently you have this problem. How long ago is " recently" ?
Do you remember a date when you did NOT have this problem?
If yes, please do a system restore. Pick a that date or even earlier as your...