I don't recall the exact 'getenv' format, but the variable you're looking for is USERPROFILE
to which you can append /My Documents
or /Documents
as necessary. Since this query is specific to Windows, ascertaining the appropriate version should be trivial.
(after a quick search) it seems to be
import os
mydocuments = os.getenv('USERPROFILE') + '/My Documents'
Note that HOMEPATH may also work (see first comment to your original query)
[edit]
also note that os.path.expanduser('~')
returns the \Documents and Settings\username
directory under windows, unix and mac osx (but not mac os) making it far more convenient when you move to another os/architecture
[/edit]
Why the downvotes, BTW? Since the first comment does not appear to be valid, is it only because of the miss between 'My Documents' and 'Documents'?