Next up is Grif's advice. And before you tell my you can't download, see what Grif tells us to do about that.
Starting a couple of weeks or so ago, I first noticed that Microsoft Security Essentials and one or two other anti-virus programs were no longer on my hard disk, then found I could not download any programs from the Internet. Always got that message "windows cannot accees the specified path..., etc." Tried three times to restore, each time a little further back but was told the restoration had failed. Thought it was AVG, but that program has been isolated and still have the problem.