You could run testdisk on the drive image to see if it can find your NTFS/FAT Windows partition, and if it can, it can rebuild the partition table for you. And if I remember correctly, you don't even need to mount the drive image as a loop device, you can simply do:
sudo testdisk sda.img
If you are using testdisk version 6.9 or greater, choose "No Log", select the drive image and "proceed", "Intel", "Analyze", "quick search", Y/N depending on if there are any Vista partitions on the drive, hit enter to continue, and select "Deeper Search" so it does a deeper search, which could take a while. It's the deeper search results that you are after if you want the best accuracy and chance of recovering the original partition(s). Once testdisk completes the deeper search and presents you with a list of partitions, select each one in turn, press "p" to get a directory listing, and any partitions that give you a directory listing are the ones you could recover. Good luck and let us know how it goes.