One thing almost always works for me... Back up all the data, drop the partitions on the hard drive, cold boot, add partion(s), boot, format and install your OS and everything else again. This is the last ditch cure all. Use sparingly. One more thing, install and update your virus software BEFORE restoring anything.
Or you can keep guessing and trying different things... The key question to ask is what changed since it last worked? Next question... Who may have changed something on your computer?