Return windows user in Delphi XE2

Following function should return the username of the currently logged in Windows user.
But when executing this code in Delphi XE2 I get a Memory Overrun leak error at line:
FreeMem(PC);

How should the code be corrected?
The function must work with Windows XP, Vista, 7 and 8
And on Delphi XE2 32bit and 64bit.

function ReturnUser:String;
var
  PC: PChar;
  Car: Cardinal;
begin
  Car := 0;
  PC := Nil;
  GetUserName(PC,Car);
  If Car <> 0 then
  begin
    GetMem(PC,Car);
    GetUserName(PC,Car);
    Result := String(PC);
    FreeMem(PC);
  end else
    Result := 'UNKNOWN';
end;

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