In addition to installing the program, you need to ensure it is updated monthly. Most companies provide free updates to their products. Update Monthly or Twice Monthly. It has been said over 600 new virus's a day come out. Check your disks for errors.
Errors occasionally happen in the file and directory systems.
If they are not found and corrected, they can grow and can potentially disable a system. Scandisk will check your disk drives for errors. If errors are found, it will allow you to repair them, at the same time possibly warning you that your hard drive may be nearing the end of its life. I have a program called Norton Utilities, that seems to be much better utilities maintence program. Do this at least monthly.