|The file could be missing from an improper shut down..........|
Or....you got hold of a program that searches out and disables anti virus programs.
The most popular programs are targets.
The first thing I would do is do a google search for free online antivirus scans.
Scan your system online.
The reason being, if you just download and install a new anti virus program.....the virus could detect it and disable the program or keep it from installing.
Then uninstall your present antivirus program and reinstall a fresh copy.