In my little corner of the world, during the last year (give or take) Hyper-V went from something that a few people were playing with to something that every single person couldn't live without. The huge benefit to us is the 'snapshot' feature, which allows us to instantaneously get a test machine back to clean state.
This left me with one problem - what to do with my old test machine which was too old to support Hyper-V, but too new to just throw away. Fortunately, Win7 came to my rescue by adding support for booting off a VHD (virtual hard disk). Now I can use differencing virtual hard disks, as a poor-man's Hyper-V and still get some millage out of my old test boxes.
Basic reimaging procedure:
Here is how I set this up: