How do I press Ctrl+Alt+Delete in VirtualBox?
Usually there is an entry in a menu somewhere. Host key+Del1 should work too, though.
If you're using Remote Desktop, then use Ctrl+Alt+End instead.
1 You should know what your host key is, but it's configurable. Mine usually was Right Ctrl.
If you want to know what your Host Key is, open VirtualBox and go to File → Preferences → Input. If you don't like the current key, you can change it to whatever you wish.
On an OS X host, try
control+alt/option+fn+delete or
cmd+fn+delete
I used the onscreen keyboard in Windows 7 which showed the guest OS receiving ctrl and alt/option, but not delete. By adding the fn key to the combination, it seems the Mac's idea of "delete" (normally like Windows' "backspace") becomes Windows' "delete".
In the menu called Input, click Keyboard then Insert Ctrl+Alt+Del:
A reminder of the shortcut is also visible there.
Note: In older versions it used to be in the Machine menu.