Actually it would be illegal, courtesy of the DMCA, which doesn't care a jot for your rights as a consumer. There is, under the DMCA, never any justification for the use of an "illegal circumvention device", and such would be required as you would have to crack the copy protection).
I'm afraid to say that I don't believe that such an "illegal circumvention device" exists for the divx encryption, I suspect the format wasn't popular enough to be worth cracking. Indeed one of the pages at "The Divx owners association" http://members.shaw.ca/the-doa/Pages/DoaDivxResources.html
states categorically that the encryption was Triple-Des, and remains uncracked. You've probably already discovered googling for such a tool is doomed to failure thanks to the identically named and very popular video codec, googling for "Circuit City divx" was more focused but didn't turn up anything useful that I could see.
I hope that you have learned a valuable lesson about DRM, and why smart consumers don't buy DRM media (unless it has already been cracked, and even then only if there is no alternative).
Dan "Rubbing salt in wounds is my speciality" Smart