In Vista, the SID seems to be located here:
Key named V, long one, bit at the end is the SID.
talks about Windows XP:
"NewSID starts by reading the existing computer SID. A computer's SID is stored in the Registry's SECURITY hive under SECURITY\SAM\Domains\Account. This key has a value named F and a value named V. The V value is a binary value that has the computer SID embedded within it at the end of its data. NewSID ensures that this SID is in a standard format (3 32-bit subauthorities preceded by three 32-bit authority fields).
Next, NewSID generates a new random SID for the computer. NewSID's generation takes great pains to create a truly random 96-bit value, which replaces the 96-bits of the 3 subauthority values that make up a computer SID. "