is it possible to freeze and unfreeze the pc screen via visual basic?

Use LockWindowUpdate API:

Public Declare Function LockWindowUpdate Lib "user32" (ByVal hwndLock As Long) As Long

Pass the hwnd of the window to be freezed to freeze the window:
LockWindowUpdate Me.Hwnd

Pass 0 to unfreeze the window:
LockWindowUpdate 0

i understand to freeze an identified window, but when you don know what window the users open, need to freeze de entire (full) screen. is this posible?

Declare a global form variable. Now whatever form a user opens set the value of that global variable to that form and pass the Hwnd of the global variable to the above method

This is not possible but if you just wanted it to look like it was frozen you could create a form that is the same size as the screen, make it always on top and bitblt the screens contents onto the form....