Administrative Tools missing

I have a computer with several problems:

1.) It is randomly getting stop errors
2.) Some programs crash upon running
3.) The administrative tools folder is blank
4.) The virus protection shield is missing from the taskbar
5.) I can not update the virus definitions

I’m sure this computer has a virus but I do not know which one. I have searched Symantec and Google and can not find any similar descriptions of viruses. Does anyone know of these problems that they have seen before?