Xbox Wire has announced that their Games for Windows Live Marketplace is shutting down as of August 25, 2013.
Those who have outstanding Microsoft Points are “encouraged” to spend it. You won’t be able to purchase new games but “you can continue to enjoy previously purchased content by downloading it through the Games for Windows Live client software as usual”.
What happens to my remaining Microsoft Points? Can I spend them?
Microsoft Points will be retired as part of the next Xbox 360 system update. The next time you initiate a purchase or redeem a Microsoft Points card or code on your console with your Microsoft account, we’ll add an amount of currency equal to or greater than the Marketplace value of your Microsoft Points to your account. You can then use the currency in your Microsoft account to buy a variety of Xbox content from your favorite Xbox stores, and if you have a Windows Phone 8 device, content from the Windows Phone Store.
As of today, all Games for Windows Live titles that have been published by Microsoft in the past are being removed from retailers. Age of Empires Online and Microsoft Flight have already been removed from Steam.
There are still some titles that have been left behind, like Fable III, Iron brigade and Gears of War. These may be removed in the days to come.
The FAQ over at Xbox.com has a detailed portion over any questions you might be having.
Games for Windows Live service itself is rumored to shut down on July 1, 2014.