My Mac doesn't boot up, it flashes a question mark on a file.

My macbook was working fine, but then i installed a torrent of ilife. the applications worked and everything seemed ok, but next time i turned on my laptop it shows a question mark on a file and also a 'blocked' sign or a no smoking logo without the cigarette. it will just keep on doing this until i turn it off.

i've tried repairing the idsk, resetting pram and disconnecting cables, but nothing works. i would be very grateful for help, as i really dont want to lose my files!

thanks a lot for any help, i appreciate it.