actually.. i meant the "window key" + "L" instead of ctrl +
L
used to lockup window.
actually, there is alot of key binding that most ppl dun
used....
mostly ppl only used "ctrl - alt - delete"
one key i found usefull is "window key" + "m"....
it minimise all app in the desktop, ie, same as show
desktop.... good when boss suddenly appear and u dun wanna get
caught surffing porn... 
the other one is "ctrl+shift+esc", bring up the task
manager....
next is the alt + tab or alt + shit + tab......
still got alot more....
Alt + Tab is by far the best.
if you use other techniques to minimize all windows, and your
boss comes over to see your desktop, obviously he knows something
is fishy.
best tactic is to open up lots of windows so that the panels on
your taskbar is completly full and overflows to 2nd row, with the
(Up and Down mini-scrollbars) at the right of your taskbar.
then, if boss comes, do Alt + Tab to switch into your whatever
spreadsheet app and pretend to be very busy. when he leaves, Alt +
Tab back into website.
if you do it right, whole process takes just 0.5 seconds and is
foolproof, unless he grabs your mouse and use the PC. Alt + Tab
method mentioned above is lot faster than any mouse can move.
Ctrl + Alt + Del is good mainly for Firefox browsers, because if
you use Ctrl + Alt + Del to forcefully close Firefox, u can restore
it to it's previous state after your boss has left, and saves
time.
meaning, i open up 7 tabs showing 7 different threads at
SGF.
when boss come, i use keyboard to Ctrl + Alt + Del to close
Firefox.
after he leaves, i run Firefox again and the 7 different threads
at SGF are all intact, no need to waste time to type in the URL or
navigate again.