Q> Как SID привести к текстовому виду(например для загрузки HKEY_USERS)?
A> См.исходник:

// nearly straight from the SDK
BOOL Sid2Text( PSID ps, char *buf, int bufSize )
{
PSID_IDENTIFIER_AUTHORITY psia;
DWORD dwSubAuthorities;
DWORD dwSidRev = SID_REVISION;
DWORD i;
int n, size;
char *p;

// Validate the binary SID.

if ( ! IsValidSid( ps ) )
return FALSE;

// Get the identifier authority value from the SID.

psia = GetSidIdentifierAuthority( ps );

// Get the number of subauthorities in the SID.

dwSubAuthorities = *GetSidSubAuthorityCount( ps );

// Compute the buffer length.
// S-SID_REVISION- + IdentifierAuthority- + subauthorities- + NULL

size = 15 + 12 + ( 12 * dwSubAuthorities ) + 1;

// Check input buffer length.
// If too small, indicate the proper size and set last error.

if ( bufSize < size )
{
SetLastError( ERROR_INSUFFICIENT_BUFFER );
return FALSE;
}

// Add 'S' prefix and revision number to the string.

size = wsprintf( buf, "S-%lu-", dwSidRev );
p = buf + size;

// Add SID identifier authority to the string.

if ( psia->Value[0] != 0 || psia->Value[1] != 0 )
{
n = wsprintf( p, "0x%02hx%02hx%02hx%02hx%02hx%02hx",
(USHORT) psia->Value[0], (USHORT) psia->Value[1],
(USHORT) psia->Value[2], (USHORT) psia->Value[3],
(USHORT) psia->Value[4], (USHORT) psia->Value[5] );
size += n;
p += n;
}
else
{
n = wsprintf( p, "%lu", ( (ULONG) psia->Value[5] ) +
( (ULONG) psia->Value[4] << 8 ) + ( (ULONG) psia->Value[3] << 16 ) +
( (ULONG) psia->Value[2] << 24 ) );
size += n;
p += n;
}

// Add SID subauthorities to the string.

for ( i = 0; i < dwSubAuthorities; ++ i )
{
n = wsprintf( p, "-%lu", *GetSidSubAuthority( ps, i ) );
size += n;
p += n;
}

return TRUE;
}