When working in the Windows installer, I've detected that threre are some problems with back & forward slashes in the installer:

dirroot is calculated with dirname() that uses "native" windows backslashes.
dataroot is calculated with moodle stuff, that always use forwardslashes

Later, dirroot is written with forwardslashes.

And then, later, in admin/index.php, dirroot is checked again with realpath(), and that doesn't pass the test when comparing back & forward slashed dirroots.

So I'm going to:

0) After chat with Sam, use always forward-slashes seems to be ok.
1) Change install to suggest forward-slashed dirroot.
2) Change admin/index.php check to ignore back & forward slashes differences.

Ciao :-)