As they say, it might be malware so if you still can't fix it run these two programs just to see what is around:
ADWCleaner (blue download button at top)
"Save" the file then double click and run the Scan. There are options but mostly it is safe to then run the Clean.
MalwareBytes free version (green button top right):
Install program and run it. Under "Settings > Detection and Protection", put a check mark in "Scan for rootkits" before running the Threat Scan from the Dashboard.
If either of them find something please copy / paste the appropriate log(s) on here in case further investigation is needed.
Always pop back and let us know the outcome - thanks