<p>Is it possible to re-assign the <kbd>Win</kbd>+<kbd>L</kbd> hotkey to another executable/shortcut?</p>

<p>Use-case - I would like to switch off the monitor of my laptop as soon as it is locked. I know of a executable which can lock and turn off the monitor but I do not want to change the way the system is locked (by running the program explicitly or by some other shortcut). It would be best if <kbd>Win</kbd>+<kbd>L</kbd> can be assigned to this executable.</p>

Looks like you can't.
You can disable all built-in Windows
  hotkeys
except Win+L
and Win+U by
  making the following change to the
  registry (this should work on all OSes
  but a reboot is probably required):
HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Policies\Explorer
  NoWinKeys REG_DWORD 0x00000001 (1)
(
http://www.autohotkey.com/docs/misc/Override.htm
)
But you could try using Tweak UI. Under the Explorer tree view item, uncheck "Enabled Windows+X" hotkeys.
Hoekey
also might work, haven't tried it.
Source
.