You should use os.path.join for this task to make your code (more) portable.
You should also use a
Popen object from the subprocess module rather than os.system for the same reason.
You could also, as a matter of preference, prepend your paths with r to create raw strings so that you don't have to escape backslashes (unless the last character in the string is a backslash, in which case it must be escaped).