I found two solutions:
1 (from: http://www.realvnc.com/products/viewerplus/1.0/docs/af1069655.html)
Configuring your keyboard
By default, and with the exception of CTRL-ALT-DELETE and the function
key used to open the shortcut menu, key presses affect the host
computer and not the client. To reverse this behavior for the
application-level keys listed below, turn off Pass special keys
directly to VNC Server. Note this property is on the Inputs tab.
Affected keys/combinations: WINDOWS (also known as START), PRINT
SCREEN, ALT-TAB, ALT-ESCAPE, CTRL-ESCAPE.
2 (from: http://www.physnet.uni-hamburg.de/physnet/vnc/winvncviewer.html)
Keystrokes such as Ctrl-Esc and Alt-Tab may be interpreted at the
local (viewer) machine. If you want to send them to the remote
machine, you can use the options on the viewer menu to send individual
Ctrl-down, Ctrl-up, Alt-down and Alt-up keystrokes. For example, to
type Ctrl-Esc on the remote machine, send Ctrl-down using the menu,
press Esc, and then send Ctrl-up (or just tap the Ctrl key) to release
the Ctrl key at the remote end.