function ConvertSidToStringSidA(Sid: PSID; var StringSid: PChar): Boolean; stdcall;  external 'ADVAPI32.DLL' name 'ConvertSidToStringSidA';

function SidToString(Sid : PSID) : string;
var
   Buffer: PChar;
begin
   if ConvertSidToStringSidA(Sid, Buffer) then
      Result:=string(Buffer)
   else
      Result:='';
end;

function GetUserSid(const MachineName, UserName: string): string;
var
   Sid: PSID;
   RefDomainName: PChar;
   SidNameUse: SID_NAME_USE;
   dwSidSize, dwDomainSize: DWORD;
begin
   Result:='';
   dwSidSize:=0;
   dwDomainSize:=0;
   if not LookupAccountNameA(PChar(MachineName), (PChar(UserName)), nil, dwSidSize, nil, dwDomainSize, SidNameUse) and (GetLastError() = ERROR_INSUFFICIENT_BUFFER) then
   try
     GetMem(Sid, dwSidSize);
     GetMem(RefDomainName, dwDomainSize);
     if LookupAccountNameA(PChar(MachineName), (PChar(UserName)), Sid, dwSidSize, RefDomainName, dwDomainSize, SidNameUse) then
        Result:=SidToString(Sid);
   finally
     FreeMem(Sid, dwSidSize);
     FreeMem(RefDomainName, dwDomainSize);
   end;
end;