test_popen fails in 2.5c2. The reason is that popen invokes

cmd.exe /c "c:\program files\python25\python.exe" -c
"import sys;print sys.argv"

cmd.exe does not support that syntax, and gives an
error (which silently disappears); the pipe read then
returns an empty string.

This problem exists atleast since Python 2.3.

To fix this, cmd.exe needs to be invoked as

cmd.exe /c "c:\program files\python25\python.exe" -c
"import sys;print sys.argv"

The attached patch fixes this by always wrapping the
command line with an addition pair of quotes.

It's not clear to me whether this can go into 2.5.1: an
application may already work around this problem by
passing extra quotes to popen, which would then break
if popen adds even more quotes.

I don't see a difference between your two command lines here... did you mean to add additional quotes in the second example?

Indeed, this should read

cmd.exe /c ""c:\program files\python25\python.exe" -c "import sys;print sys.argv""

I applied the change to:

Python 2.6a0 (trunk:58651M, Nov 18 2007, 08:46:54) [MSC v.1400 32 bit
(Intel)] on win32

and test_popen passes appeared to pass.

subprocess also needs this fix applied

Does the w9xopen command line below not need this?

What is needed is separate quoting for the command and the argument. 
Right now, test_popen only surrounds the entire command line with quotes:

"c:\Program Files\Python2.6\Python.exe -u c:\Documents and Settings\Russ
Gibson\cgi-bin\cgi.py"

It needs to the above as thus:

"c:\Program Files\Python2.6\Python.exe" -u "c:\Documents and
Settings\Russ Gibson\cgi-bin\cgi.py"

From the command line, the second case works, but the first doesn't. 
Neither work from os.popen in 2.6.2.

To make sure that popen works in all cases under windows, test_popen*
need to test with separate quotes around the command and argument.  That
will show the real problem with the current os.popen* implementation(s).

(yeah, I know, shut up and provide a patch...)

This has come up recently and Martin's approach seems to work. I updated his patch for trunk, and test_popen passes when I run it with and without a space in the path. 

Russ Gibson's suggestion was already applied to test_popen, so the only code change is to posixmodule.c.

Here is a py3k version of the previous patch. os.popen is implemented using subprocess.Popen, so the quoting change was made there.

It is on Python 2.7a2

>>> os.popen('"C:\\Python27\\python.exe" -c "import sys; print sys.argv" 42').read()
''

>>> os.popen('""C:\\Python27\\python.exe" -c "import sys; print sys.argv" 42"').read()
"['-c', '42']\n"

>>> os.popen3('"C:\\Python27\\python.exe" -c "import sys; print sys.argv" 42')[2].read()
'\'C:\\Python27\\python.exe" -c "import\' est pas reconnu en tant que commande interne ou externe, un programme executable ou un fichier de commandes.\n'

It may be related to the test_popen failure.
Actually, it seems that the outer double quotes are removed before executing the cmdstring:
  C:\Python27\python.exe" -c "import
 ^                                  ^

Florent is correct. The patch seems to fix regular popen, but popen3 sees problems. I'll see if I can fit this in and have a look.

Also of note is that the other flavors of popen are not tested...at least not in Lib/test/test_popen.py or Lib/test/test_os.py