Problems running MPICH2

[Marcos Bahia]

I'm a very new user to MPI and I have been having some problems. I
hope you can help me.
I have installed MPICH2 in a network of PCs. The processors are
Pentium 4 - 2,60 GHz and the operational system is Windows 2000
Professional Edition. I had no problems while installing MPICH2 but I
got some strange results while running CPI.exe.
I used the command "mpiexec –localonly 1 cpi.exe" and got the
following results for n = 10000000:

Pi is approximately 3.1415926535897309
Error is 0.0000000000000622
Wall clock time = 0.146069

So I began to increase the number of PCs involved in the computation
and the time, as expected, decreased accordingly, leading to a very
linear behavior. I kept only one process running in each machine.
The problem was that, keeping n = 10000000, the error returned was
different every time a new computation with an additional PC was run.
The results are:

2 PCs -> Error = 0.0000000000001918, wall clock time = 0.073221;
3 PCs -> Error = 0.0000000000001190, wall clock time = 0.049889;
4 PCs -> Error = 0.0000000000001066, wall clock time = 0.037724;
5 PCs -> Error = 0.0000000000000786, wall clock time = 0.030447;
6 PCs -> Error = 0.0000000000000226, wall clock time = 0.025617;
7 PCs -> Error = 0.0000000000000027, wall clock time = 0.022449;
8 PCs -> Error = 0.0000000000000138, wall clock time = 0.020213.

The network is connected through a 3Com, 16 port switch.
Any idea of what might be happening?