Microsoft has recently revealed that they are closing down the Xbox.com PC Marketplace. Although you will not be able to purchase new games, you can continue to play previously purchased content by downloading it through the Games for Windows Live client software as usual. Also the the Games for Windows Live service will continue to operate as usual.
As part of the recent Xbox 360 system update, Microsoft Points will be retired and the Xbox.com PC Marketplace will be closed as of August 22, 2013. We encourage you to spend your Microsoft Points balance prior to this change.
This is not a surprise move from Microsoft as they continue to focus on Xbox for games and entertainment.