My computer won't start

Hello,
i have a acer windows vista computer that wont boot up. i have two login screens the first one requires a different password than the regular windows window. the problem is , I received a notice on my avpst antivirus that it stopped a trojan viruses.I tried to conain the virus and it would'nt put the virus in a chest. I tried it again and the computer turned off and came back on with about ten options , the second option was delete all. which I believe deleted all the files and drivers off of the computer, because now the computer only goes to the first two sign in screens. then turns off and turn itself back on to the signon screen again. help!!!!!!!!!!