UltraVNC can't send Ctrl-Alt-Del

http://kb.realvnc.com/questions/102/

reg add HKLM\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System /v SoftwareSASGeneration /t REG_DWORD /d 1 /f

This registry value allows UltraVNC (and others) to send Ctrl-Alt-Del through the installed service. It is also configurable as a policy in gpedit.msc,
.. Computer Configuration
.... Administrative Templates
...... Windows Components
........ Windows Logon Options
.......... Disable or enable software Secure Attention Sequence
............ Enable for "Services".