No, MPI_COMM_SIZE doesn't tell anything about the number of processors, under either of the common definitions. Core 2 Duo is certainly capable of running 2 MPI ranks, or 2 OpenMP threads, even under XP32. Did you start your job by mpirun -np 2 or similar ?
MPI implementations, in my experience, never pick a number of ranks automatically, unlike OpenMP, where some implementation default to 1 thread, and others default to total number of cores or logicals.
Intel like to call a Core 2 Duo a single processor dual core, even though the term logical processors also is used, particularly for HT. Many MPI users consider dual core as 2 processors.
