os.popen() not executing command on windows xp

[nic]

On my system (WinXP) typing the following line into command
prompt(cmd.exe) successfully scans the file test1.txt:
"c:\Program Files\Grisoft\AVG Free\avgscan.exe" "c:\program
files\temp1\test1.txt"

Yet the python script:
import os
a = os.popen('"c:\Program Files\Grisoft\AVG Free\avgscan.exe"
"c:\program files\temp1\test1.txt"')
print a.read()

Returns a blank line, and I doesn't scan the file. (Note I've used
os.popen() successfully on my system in other situations like:
os.popen('taskkill /F /IM taskname.exe')).

I have a feeling my avgscan example shown above not working has
something to do with calling a 3rd party software program (avgscan.exe)
through popen, but I don't know why this won't work, when manually
typing it in the command line does?