> "early" is not correct: it's late in fact, only when the desktop is opened. I should use GPEDIT.MSC to start a task before the desktop, but this command doesn't seem to be available on my Windows 8?

Oh, I found how to start a task before the user login, and os.urandom() still works: it produces 16 bytes immediatly (in 0.0 second).

As FreeBSD, I'm not sure that Windows is really impacted by the issue #26839.

Latest article about Windows RNG: http://eprint.iacr.org/2007/419
"Cryptanalysis of the Random Number Generator of the Windows Operating System" (2007) by Leo Dorrendorf and Zvi Gutterman and Benny Pinkas

See also "Where does a Hyper-V guest get its entropy when generating a certificate authority key pair?":
http://security.stackexchange.com/questions/51538/where-does-a-hyper-v-guest-get-its-entropy-when-generating-a-certificate-authori