Another option which can work extremely well (but has a rather large initial learning curve) is to use a source code control system.
Usually used for software development, they can be used to keep versions of any files, not just software source code.
There are several free options (some are free only for 1 or 2 users, but in this context that's generally not a problem):
CVS: used by much of the open source software world http://ximbiot.com/cvs/
TortoiseCVS: a better interface for CVS (integrates into Windows Explorer) http://www.tortoisecvs.org/
Subversion: an up-and-coming alternate to CVS http://subversion.tigris.org/
TortoiseSVN: a better interface for Subversion (integrates into Windows Explorer) http://tortoisesvn.net/
(free for up to 2 users)
(free for a single user)
This is just a small sample of some of the bigger players; there are many others.
I'd suggest one of the TortoiseXxx versions - they're pretty easy to use once you get things set up and you've used it on a couple files, they're free, and they're widely used with a ton of info and help out in the Google-Sphere.
Once you're using one of these systems, you can always just work on the most recent file and save that file to the version control system periodically, but at anytime you can get any previous version.
One nice thing that Perforce has is an MS Office plug-in - I'm surprised this hasn't happened for CVS or Subversion, yet.