TweetDeck has announce that it will discontinuing the Windows desktop app for TweetDeck. Going forward, Windows users will have to access TweetDeck from their browsers, by visiting tweetdeck.twitter.com.
The desktop apps for Windows and OS X were created after Twitter acquired TweetDeck back in 2011. They were nothing more than the website inside a wrapper, so there wasn't a lot of reasons to use them to begin with.
Incidentally, the OS X app and the Chrome "app" continue to exist for now, and it's just the Windows app getting the axe. Perhaps the OS X app will be dropped soon too, since both platforms have native apps from Twitter now on their respective stores.
Twitter has also made it easier to log into TweetDeck now. If you're logged into the Twitter website in your browser, you will automatically be logged in to your TweetDeck.