From 16th Feb 2015, Google have decided you must use their Chrome extension to access Google Talk (now rebranded "Google Hangouts").
This means running Chrome in the background, and installing the Google Hangouts extension into Chrome.  You'll have 2 system tray icons; the Chrome one,
and the Google Hangouts one.
It sucks, but Google have shoved this down our throat.  There's no choice to continue using this old GTalk program.

Install the Google Hangouts extension from:
  https://chrome.google.com/webstore/detail/hangouts/nckgahadagoaajjgafhacjanaoiihapd

Note that it should be called "Hangouts" and it should be an *extension* and not an app.  Some text indicating that it's an extension should be on the page,
like "USERS OF THIS EXTENSION HAVE ALSO USED...".  Google also make an "app" called "Hangouts" which puts a dumb icon on the right of the screen,
apparently designed primarily for the Chromebook interface.  You don't want this.  Install the *extension* instead to have the icon go in the systray.

For reference, the "app" version (which you don't want) is at:
  https://chrome.google.com/webstore/detail/hangouts/knipolnnllmklapflnccelgolnpehhpl

One thing I've found is that Chrome doesn't tend to stay running in the background.  It shuts down from time to time.  Therefore, you should also copy the
included ChromeMonitor.exe I created into something like a StandaloneApps directory, and set it to run on startup.  On first run, it will write a sample
config file that will need to be edited to point to your Google Chrome executable.

See the systray.png image to see what icons should appear in the system tray on startup once this is all set up.