Hmm that's odd.

A single '.' represents the current working path. So you should in theory use that, e.g:
the above code would open a file called test.txt in the SubFolder, which would be located in the same folder as the 'current working directory' (which SHOULD be where the script is).
So the full path for test.txt could be something like:

"Python/SubFolder/"

But I'm not sure why the previous example didn't work.
Anyway, try using a . in your path, that may work.

hope that helps
alex

P.S
By the way, a single dot (.) means the current folder,
a double dot (..) would go back one folder so you could use "../../" to go back two folders.

EDIT:
One more thing, http://www.bembry.org/technology/pyt.../lesson_12.php
has some pretty good info basic file/folder manipulation.