How do I get my computer to download/open .exe files again?

When i go to open a program from my desk top or anywhere, I get an error message that this program does not have a file ociated with it to perform that action. Creat an ociation in Folder Options contro panel. I believe I must have gotten a virus, however I can't download a program to remove it and I can' run what I have. I can't even open add/remove program icon in control panel. Please help