lock pc just by double clicking mouse

You can lock your XP workstation with two clicks of
the mouse. Create a new shortcut on your desktop using
a right mouse click, and enter 'rundll32.exe
user32.dll,LockWorkStation' in the location field.
Give the shortcut a name you like. That's it -- just
double click on it and your computer will be locked.
And if that's not easy enough, Windows key + L will do
the same.