Flashing taskbar with PuTTY + irssi + screen

In Irssi:
  /set beep_when_window_active ON
  /set beep_when_away ON
  /set bell_beeps on
  /set beep_msg_level MSGS NOTICES DCC DCCMSGS HILIGHT

And in Putty:
  Go to Terminal -> Bell in the settings, and set "Taskbar/caption indication on bell" to Flashing or 
  Steady (whichever you prefer).

In Screen, remember to set the audible bell (unless you've already set "vbell off" in .screenrc):
  <Ctrl>-<a> <Ctrl>-<g>
or if you're already in screen, reload the .screenrc file on the fly:
  <Ctrl>+<a>+":" source $HOME/.screenrc<CR>