I reply to myself:

Qsettings documentation says it is not possible to read or write keys with backslashes:

http://doc.trolltech.com/4.3/qsettings.html

"Note that the backslash character is, as mentioned, used by QSettings to separate subkeys. As a result, you cannot read or write windows registry entries that contain slashes or backslashes; you should use a native windows API if you need to do so."