Cannot execute Windows commands via Python in 64-bit

[nsjmetzger at gmail.com (nsjmetzger at gmail.com)]

I have a script that runs fine in Windows 2003 (32-bit). It basically
calls the Windows defrag command. When I try the exact same code on
Windows 2003 (64-bit) I get the following message:

 C:\Scripts>autodefrag.py
Starting defragment:  defrag -v C: >>c:/Scripts/DEFRAG20070502.log
'defrag' is not recognized as an internal or external command,
operable program or batch file.

I have tried defrag.exe and even giving it the full path to
defrag.exe. Always the same result. Here is the python code that is
generating this error:

cmd = "defrag -v C: >>c:/Scripts/DEFRAG20070502.log"
print "Starting defragment: ", cmd
errorlevel = os.system(cmd)


Anyone know what the heck is going on and how to fix it? This code
works fine on my 32-bit windows machines.
Thanks.