As quite many people do, I use to IRC using screen + irssi on my server, which I connect to using PuTTY. A little while ago I went out to find a way to make PuTTY’s taskbar flash when I was hilighted. It wasn’t trivial to find the information on how to do it (it took me some time), so I though I could share it:

In irssi:
/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).

That should be it, and don’t forget to save the session settings in PuTTY so you don’t have to change them every time you connect ;) .