.daa files are proprietary to PowerISO and cannot be opened by any other applications. My suggestion would be to download the trial of Crossover onto the iMac
http://www.codeweavers.com/products/cxmac/ Then download PowerISO onto the iMac and install it using crossover and convert the .daa to and iso. Your other option is to purchase a licensed copy of Windows XP Pro SP2 and either purchase VirtualPC (for you powerbook) or Parallels (for the iMac), or bootcamp the iMac and install Windows that way.
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Opening a .daa file?
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