You are most likely having a virus on your system. Viruses are very difficult to cure even when you load the best anti virus after the computer has been infected. They have been designed to hide and mutate like human viruses. They change file names and hide using these new file names. Anti viruses get confused and can't clean them properly.
What usually works for me is to re-format the computer. That is to clean the hard drive and then load software afresh this time with anti virus loaded. This way the viruses are always kept at bay. They are stopped before they infect the system.