A lot of people use the newsid tool to generate new sids for servers this unfortunately does not work so well with server 2008 and can produce some stunning results if you try and use it in R2 (hint it wont boot afterwards).
However you can create a newsid with sysprep, which is installed by default with server 2k8.
Type sysprep into the box and hit enter
it will direct you to the sysprep folder
double click the exe in the folder
leave the default option “Enter system out of box experience” and tick the generalise box which is the bit that will generate a new sid.
Click OK and wait for a bit the server will reboot run though some basic installation tasks and also run you through the mini-setup process.
This is great for template vm’s that have not already been set to to use the customisation wizards available in the various hypervisors.