Im new to bitcomet..i downloaded a file which is in .daa file..i den download powerISO..so what do i do now? 
A .daa file is a proprietary type of disk image. You can use PowerISO to mount the disk in a virtual drive, or to convert it to an .iso or other image type that can be mounted by Alcohol 120% or Daemon Tools, or burned to an optical disk by most burning software.