As you didn't pass it through gzip it'll be trivial to mount -o loop under linux, so you don't need to copy it onto a disk to read data from it. Virtualbox / vmware can probably be forced to boot a virtual machine from the image; I've done that with a real partition and linux thinks everything is a file so it should be possible.
If it was a linux install, then modifying the initrd to mount the image somewhere then hand control over to a kernel found on that image will be possible but irritating to implement.
Booting an image of a windows install from a file is unlikely to be possible. Or at least, not without silly amounts of work. If you manage it then all the people who would like to boot windows in a ramdisk would love to hear from you.