Cant find answer! Please help!

Hello, i think i got a really bad virus! I restarted my computer and now it won't start. I have an acer laptop with windows vista, no boot cd, when i turn on the computer it gives me 2 options, launch startup repair (recommended) and start windows normally, well windows will mot start normally, it just takes me to a black screen with a mouse then restart again, if i choose startup repair it take me to what looks like a startup screen but only gives me the option of 'other user' but it has no username or password??? It won't start in safe mode at all, and i tried last known config but nothing is working!! Please Help Me!!!