From
http://technet.microsoft.com/en-us/sysinternals/bb897418.aspx
Duplicate SIDs aren't an issue in a Domain-based environment since domain
accounts have SID's based on the Domain SID. But, according to Microsoft
Knowledge Base article Q162001, "Do Not Disk Duplicate Installed Versions
of Windows NT", in a Workgroup environment security is based on local
account SIDs. Thus, if two computers have users with the same SID, the
Workgroup will not be able to distinguish between the users. All
resources, including files and Registry keys, that one user has access to,
the other will as well.
So, from the way I read it the computer's SID doesn't change when added to
a Domain, but Domain accounts get SIDs based on the Domain SID so
duplicate computer SIDs aren't a problem in a Domain. In a workgroup on
the other hand, it is a problem.
Hope this helps!
--
Zaphod
Arthur: All my life I've had this strange feeling that there's something
big and sinister going on in the world.
Slartibartfast: No, that's perfectly normal paranoia. Everyone in the
universe gets that.