Running an outside program (executable) in Python?

I just started working on Python and have been trying to run an outside executable from Python.

I have an executable for a program written in Fortran. Let’s say the name for the executable is flow.exe, and my executable is located in C:\Documents and Settings\flow_model. I tried both os.system and popen commands, but so far, I couldn't make it work. The following code seems like it opens the command window, but it wouldn't execute the model.

# Import system modules
import sys, string, os, arcgisscripting
os.system("C:/Documents and Settings/flow_model/flow.exe")

How can I fix this?