Q> Как узнать SID юзера?
A> Из исходника getadmin:

BOOL
GetAccountSid(
LPTSTR SystemName,
LPTSTR AccountName,
PSID *Sid
)
{
LPTSTR ReferencedDomain=NULL;
DWORD cbSid=128; // initial allocation attempt
DWORD cbReferencedDomain=16; // initial allocation size
SID_NAME_USE peUse;
BOOL bSuccess=FALSE; // assume this function will fail

__try {

//
// initial memory allocations
//
if((*Sid=HeapAlloc(
GetProcessHeap(),
0,
cbSid
)) == NULL) __leave;

if((ReferencedDomain=(LPTSTR)HeapAlloc(
GetProcessHeap(),
0,
cbReferencedDomain
)) == NULL) __leave;

//
// Obtain the SID of the specified account on the specified system.
//
while(!LookupAccountName(
SystemName, // machine to lookup account on
AccountName, // account to lookup
*Sid, // SID of interest
&cbSid, // size of SID
ReferencedDomain, // domain account was found on
&cbReferencedDomain,
&peUse
)) {
if (GetLastError() == ERROR_INSUFFICIENT_BUFFER) {
//
// reallocate memory
//
if((*Sid=HeapReAlloc(
GetProcessHeap(),
0,
*Sid,
cbSid
)) == NULL) __leave;

if((ReferencedDomain=(LPTSTR)HeapReAlloc(
GetProcessHeap(),
0,
ReferencedDomain,
cbReferencedDomain
)) == NULL) __leave;
}
else __leave;
}

//
// Indicate success.
//
bSuccess=TRUE;

} // finally
__finally {

//
// Cleanup and indicate failure, if appropriate.
//

HeapFree(GetProcessHeap(), 0, ReferencedDomain);

if(!bSuccess) {
if(*Sid != NULL) {
HeapFree(GetProcessHeap(), 0, *Sid);
*Sid = NULL;
}
}

} // finally

return bSuccess;
}