VNC can't send a Ctrl-Alt-Delete in Windows 7

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Manually, try the below.

1) "Start menu" ->type  "Execute" without the quotes - > gpedit.msc
2) Navigate to "Computer Configuration" -> "Administrative Templates" -> "Windows Components" -> "Windows Logon Options"
3) Double-click on the "Disable or enable software Secure Attention Sequence" parameter.
4) Check "Enable", then select "Services" in the combobox.
5) Apply the modification