My CPU utility is about 5% under windows XP sp2 environment on 4 computers. Memory of each computer is 1GB. To enchance CPU utilization, what should be noticed when programming with mpi?

Probably your program has been stalling in some MPI communication waiting some feedback from other processes. It can be also due unbalanced processes. For a good starting point, it would be useful to measure the wall time spent in each routine to see where your bottleneck is. You can easily do it issuing MPI_WTIME callings (or cpu_time if you're working in Fortran) before and after your routines and accumulating and printing the time spent.

Cheers

Renato.