Dear Valeria,
There is an example on Developer Zone which provides a list of all Windows printers installed on your computer i.e. the same information that you would see if you clicked on Start>>Settings>>Printers. The following link provides more information:
http://zone.ni.com/devzone/explprog.nsf/6c163603265406328625682a006ed37d/9c427847a217807a8625695000652ae7?OpenDocument
I'm not sure if this tells you the currently selected printer - as I haven't been able to run the sample code before submitting my answer. Nor will it tell you the printer selected by the user. However, it may provide some help to understand how you can solve your problem.
The registry holds information regarding the current selected printer and as an alternative you could us
e the Registry VIs (introduced with LabVIEW 6.0?) to interrogate the appropriate key.
Jeremy
