I make heavily use of virtual machines. For development, testing beta applications (like Visual Studio 2010) and client-side testing. When something goes wrong, you just abandon the image, get a fresh copy and start again.
One of the things I always used to do is run the NewSID tool from sysinternals to generate a new SID for the copied VM. There was something about problems when running multiple computer with the same SID. At least, that was what I was told a long time ago by a system administrator. And they should know, right?
Wrong. It seems the whole process of changing a computers SID was a myth. Take some time and read through the article by Mark Russinovich. He is the creator of NewSID and explains in detail why it is not necessary anymore (and never was) to change a machines SID.
The full article: Mark's Blog : The Machine SID Duplication Myth