I changed the script to use subprocess (attached file) and got the same 
rollover errors as before.  I had to change cd and del to be cd.bat and 
del.bat which contained cd %1 and del %1 respectively since it appears 
subprocess can't run internal commands like cd and del (unless you 
specify shell = True, which I thought might defeat the purpose of the 
test).

I will search around for this bug to see if it's already been entered.  
If the python developers decide not to fix this by wrapping os.system 
(and I guess subprocess.Popen too) with locks to prevent this error, 
then I agree that it should at least be well documented.