Make sure you have the .ISO image and know what directory it's under.
Open Daemon tools from a desktop shortcut or from the start menu or whatever. Note that a window won't open, but there will be a red icon in your system tray at the bottom right.
Right click the icon in the system tray. Mouse over "Virtual CD/DVD-ROM". Then mouse over Device 0, then click Mount Image. (If you don't have a Device 0, then mouse over Set Number of Drives, then select how many virtual drives you want). After you click Mount Image, search around for the image (the file in the torrent), then open that.
If nothing happens once you've done that, go to Run... under the Start menu, hit browse, go to My Computer from the browsing menu, open the virtual drive you created, then select AutoRun or Setup and open it. Then hit OK.
If you haven't downloaded the file through a bittorrent client, then I suggest you do that first.