When you start a Windows 7 or equivalent server machine, to log in, you need to press CTRL-ALT-DELETE on the keyboard to get the login page. To send Control-Alt-Delete to a Windows virtual machine in Virtual Box from a Macintosh
on a Macintosh laptop press the fn – Command 
– Delete back 
keys
on a Macintosh desktop press the Command 
– Delete forward 
keys.
I have a Dutch QWERTY numeric keyboard which appears to be slightly different from the one on the last picture and in my case the options mentioned above didn’t work, but I found out that control + option + delete forward did the trick for me.