hi,
this is how to send control + alt + delete with virtual usb keyboard library
from the code found in :
http://www.practicalarduino.com/projects/virtual-usb-keyboard
and: Google Code Archive - Long-term storage for Google Code Project Hosting.
UsbKeyboard.sendKeyStroke(76,5); // 76 is the delete key from the hid descriptor, 5 is the INTEGER combination of
MOD_CONTROL_LEFT (1<<0) and MOD_ALT_LEFT (1<<2)
all the keys of the keyboard at page 53 http://www.usb.org/developers/devclass_docs/Hut1_11.pdf
I hope it helps someone as I was stuck with this for long time 