Did you do a full virus and spyware scan? If not, do that. Avira Antivir has a rescue boot disk you can create and run on the system to do a scan without needing to boot the computer. After that runs if you can run programs after that, install and run Malwarebytes.
If this does not help, you may need to restore the system to factory defaults which will wipe your files, so hopefully you made backups or can backup the files with the system as it is.